Method for forming metal matrix composite bodies with a dispersion casting technique

ABSTRACT

The present invention relates to a novel method for forming metal matrix composite bodies. A permeable mass of filler material is spontaneously infiltrated by a molten matrix metal. Particularly, an infiltration enhancer and/or an infiltration enhancer precursor and/or an infiltration atmosphere are in communication with the filler material, at least at some point during the process, which permits molten matrix metal to spontaneously infiltrate the filler material. After infiltration has been completed to a desired extent, additional matrix metal is added to that matrix metal which has spontaneously infiltrated the filler material to result in a suspension of filler material and matrix metal, said suspension having a lower volume fraction of filler relative to matrix metal. The matrix metal then can be permitted to cool in situ or the mixture of matrix metal and filler material can be poured into a second container as a casting process to form a desired shape which corresponds to the second container. However, the formed suspension, whether cast immediately after being formed or cooling and thereafter heating and casting, can be pour cast into a desired shape.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 07/269,308, filed 11/10/88, having an issue date of Mar. 19,1991, and having a U.S. Pat. No. of 5,000,247, and naming as soleinventor John Thomas Burke and entitled "Method For Forming Metal MatrixComposite Bodies With A Dispersion Casting Technique and ProductsProduced Thereby", the subject matter of which is incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates to a novel method for forming metal matrixcomposite bodies. A permeable mass of filler material is spontaneouslyinfiltrated by a molten matrix metal. Particularly, an infiltrationenhancer and/or an infiltration enhancer precursor and/or aninfiltrating atmosphere are in communication with the filler material,at least at some point during the process, which permits molten matrixmetal to spontaneously infiltrate the filler material. Afterinfiltration has been completed to a desired extent, additional matrixmetal is added to that matrix metal which has spontaneously infiltratedthe filler material to result in a suspension of filler material andmatrix metal having a lower volume fraction of filler relative to matrixmetal. The matrix metal then can be permitted to cool in situ or themixture of matrix metal and filler material can be poured into a secondcontainer as a casting process to form a desired shape which correspondsto the second container. However, the formed suspension, whether castimmediately after being formed or after cooling and thereafter heatingand casting, can be pour cast into a desired shape while retainingbeneficial characteristics associated with spontaneously infiltratedmetal matrix composites.

BACKGROUND OF THE INVENTION

Composite products comprising a metal matrix and a strengthening orreinforcing phase such as ceramic particulates, whiskers, fibers or thelike, show great promise for a variety of applications because theycombine some of the stiffness and wear resistance of the reinforcingphase with the ductility and toughness of the metal matrix. Generally, ametal matrix composite will show an improvement in such properties asstrength, stiffness, contact wear resistance, and elevated temperaturestrength retention relative to the matrix metal in monolithic form, butthe degree to which any given property may be improved depends largelyon the specific constituents, their volume or weight fraction, and howthey are processed in forming the composite. In some instances, thecomposite also may be lighter in weight than the matrix metal per se.Aluminum matrix composites reinforced with ceramics such as siliconcarbide in particulate, platelet, or whisker form, for example, are ofinterest because of their higher stiffness, wear resistance and hightemperature strength relative to aluminum.

Various metallurgical processes have been described for the fabricationof aluminum matrix composites, including methods based on powdermetallurgy techniques and liquid-metal infiltration techniques whichmake use of pressure casting, vacuum casting, stirring, and wettingagents. With powder metallurgy techniques, the metal in the form of apowder and the reinforcing material in the form of a powder, whiskers,chopped fibers, etc., are admixed and then either cold-pressed andsintered, or hot-pressed. The maximum ceramic volume fraction in siliconcarbide reinforced aluminum matrix composites produced by this methodhas been reported to be about 25 volume percent in the case of whiskers,and about 40 volume percent in the case of particulates.

The production of metal matrix composites by powder metallurgytechniques utilizing conventional processes imposes certain limitationswith respect to the characteristics of the products attainable. Thevolume fraction of the ceramic phase in the composite is limitedtypically, in the case of particulates, to about 40 percent. Also, thepressing operation poses a limit on the practical size attainable. Onlyrelatively simple product shapes are possible without subsequentprocessing (e.g., forming or machining) or without resorting to complexpresses. Also, nonuniform shrinkage during sintering can occur, as wellas nonuniformity of microstructure due to segregation in the compactsand grain growth.

U.S. Pat. No. 3,970,136, granted Jul. 20, 1976, to J. C. Cannell et al.,describes a process for forming a metal matrix composite incorporating afibrous reinforcement, e.g. silicon carbide or alumina whiskers, havinga predetermined pattern of fiber orientation. The composite is made byplacing parallel mats or felts of coplanar fibers in a mold with areservoir of molten matrix metal, e.g., aluminum, between at least someof the mats, and applying pressure to force molten metal to penetratethe mats and surround the oriented fibers. Molten metal may be pouredonto the stack of mats while being forced under pressure to flow betweenthe mats. Loadings of up to about 50% by volume of reinforcing fibers inthe composite have been reported.

The above-described infiltration process, in view of its dependence onoutside pressure to force the molten matrix metal through the stack offibrous mats, is subject to the vagaries of pressure-induced flowprocesses, i.e., possible non-uniformity of matrix formation, porosity,etc. Non-uniformity of properties is possible even though molten metalmay be introduced at a multiplicity of sites within the fibrous array.Consequently, complicated mat/reservoir arrays and flow pathways need tobe provided to achieve adequate and uniform penetration of the stack offiber mats. Also, the aforesaid pressure-infiltration method allows foronly a relatively low reinforcement to matrix volume fraction to beachieved because of the difficulty inherent in infiltrating a large matvolume. Still further, molds are required to contain the molten metalunder pressure, which adds to the expense of the process. Finally, theaforesaid process, limited to infiltrating aligned particles or fibers,is not directed to formation of aluminum metal matrix compositesreinforced with materials in the form of randomly oriented particles,whiskers or fibers.

In the fabrication of aluminum matrix-alumina filled composites,aluminum does not readily wet alumina, thereby making it difficult toform a coherent product. Various solutions to this problem have beensuggested. One such approach is to coat the alumina with a metal (e.g.,nickel or tungsten), which is then hot-pressed along with the aluminum.In another technique, the aluminum is alloyed with lithium, and thealumina may be coated with silica. However, these composites exhibitvariations in properties, or the coatings can degrade the filler, or thematrix contains lithium which can affect the matrix properties.

U.S. Pat. No. 4,232,091 to R. W. Grimshaw et al., overcomes certaindifficulties in the art which are encountered in the production ofaluminum matrix-alumina composites. This patent describes applyingpressures of 75-375 kg/cm² to force molten aluminum (or molten aluminumalloy) into a fibrous or whisker mat of alumina which has been preheatedto 70° to 1050° C. The maximum volume ratio of alumina to metal in theresulting solid casting was 0.25/1. Because of its dependency on outsideforce to accomplish infiltration, this process is subject to many of thesame deficiencies as that of Cannell et al.

European Patent Application Publication No. 115,742 describes makingaluminum-alumina composites, especially useful as electrolytic cellcomponents, by filling the voids of a preformed alumina matrix withmolten aluminum. The application emphasizes the non-wettability ofalumina by aluminum, and therefore various techniques are employed towet the alumina throughout the preform. For example, the alumina iscoated with a wetting agent of a diboride of titanium, zirconium,hafnium, or niobium, or with a metal, i.e., lithium, magnesium, calcium,titanium, chromium, iron, cobalt, nickel, zirconium, or hafnium. Inertatmospheres, such as argon, are employed to facilitate wetting. Thisreference also shows applying pressure to cause molten aluminum topenetrate an uncoated matrix. In this aspect, infiltration isaccomplished by evacuating the pores and then applying pressure to themolten aluminum in an inert atmosphere, e.g., argon. Alternatively, thepreform can be infiltrated by vapor-phase aluminum deposition to wet thesurface prior to filling the voids by infiltration with molten aluminum.To assure retention of the aluminum in the pores of the preform, heattreatment, e.g., at 1400° to 1800° C., in either a vacuum or in argon isrequired. Otherwise, either exposure of the pressure infiltratedmaterial to gas or removal of the infiltration pressure will cause lossof aluminum from the body.

The use of wetting agents to effect infiltration of an alumina componentin an electrolytic cell with molten metal is also shown in EuropeanPatent Application Publication No. 94353. This publication describesproduction of aluminum by electrowinning with a cell having a cathodiccurrent feeder as a cell liner or substrate. In order to protect thissubstrate from molten cryolite, a thin coating of a mixture of a wettingagent and solubility suppressor is applied to the alumina substrateprior to start-up of the cell or while immersed in the molten aluminumproduced by the electrolytic process. Wetting agents disclosed aretitanium, zirconium, hafnium, silicon, magnesium, vanadium, chromium,niobium, or calcium, and titanium is stated as the preferred agent.Compounds of boron, carbon and nitrogen are described as being useful insuppressing the solubility of the wetting agents in molten aluminum. Thereference, however, does not suggest the production of metal matrixcomposites, nor does it suggest the formation of such a composite in,for example, a nitrogen atmosphere.

In addition to application of pressure and wetting agents, it has beendisclosed that an applied vacuum will aid the penetration of moltenaluminum into a porous ceramic compact. For example, U.S. Pat. No.3,718,441, granted Feb. 27, 1973, to R. L. Landingham, reportsinfiltration of a ceramic compact (e.g., boron carbide, alumina andberyllia) with either molten aluminum, beryllium, magnesium, titanium,vanadium, nickel or chromium under a vacuum of less than 10⁻⁶ torr. Avacuum of 10⁻² to 10⁻⁶ torr resulted in poor wetting of the ceramic bythe molten metal to the extent that the metal did not flow freely intothe ceramic void spaces. However, wetting was said to have improved whenthe vacuum was reduced to less than 10⁻⁶ torr.

U.S. Pat. No. 3,864,154, granted Feb. 4, 1975, to G. E. Gazza et al.,also shows the use of vacuum to achieve infiltration. This patentdescribes loading a cold-pressed compact of AlB₁₂ powder onto a bed ofcold- pressed aluminum powder. Additional aluminum was then positionedon top of the AlB₁₂ powder compact. The crucible, loaded with the AlB₁₂compact "sandwiched" between the layers of aluminum powder, was placedin a vacuum furnace. The furnace was evacuated to approximately 10⁻⁵torr to permit outgassing. The temperature was subsequently raised to1100° C. and maintained for a period of 3 hours. At these conditions,the molten aluminum penetrated the porous AlB₁₂ compact.

U.S. Pat. No. 3,364,976, granted Jan. 23, 1968, to John N. Reding etal., discloses the concept of creating a self-generated vacuum in a bodyto enhance penetration of a molten metal into the body. Specifically, itis disclosed that a body, e.g., a graphite mold, a steel mold, or aporous refractory material, is entirely submerged in a molten metal. Inthe case of a mold, the mold cavity, which is filled with a gas reactivewith the metal, communicates with the externally located molten metalthrough at least one orifice in the mold. When the mold is immersed intothe melt, filling of the cavity occurs as the self-generated vacuum isproduced from the reaction between the gas in the cavity and the moltenmetal. Particularly, the vacuum is a result of the formation of a solidoxidized form of the metal. Thus, Reding et al. disclose that it isessential to induce a reaction between gas in the cavity and the moltenmetal. However, utilizing a mold to create a vacuum may be undesirablebecause of the inherent limitations associated with use of a mold. Moldsmust first be machined into a particular shape; then finished, machinedto produce an acceptable casting surface on the mold; then assembledprior to their use; then disassembled after their use to remove the castpiece therefrom; and thereafter reclaim the mold, which most likelywould include refinishing surfaces of the mold or discarding the mold ifit is no longer acceptable for use. Machining of a mold into a complexshape can be very costly and time-consuming. Moreover, removal of aformed piece from a complex-shaped mold can also be difficult (i.e.,cast pieces having a complex shape could be broken when removed from themold). Still further, while there is a suggestion that a porousrefractory material can be immersed directly in a molten metal withoutthe need for a mold, the refractory material would have to be anintegral piece because there is no provision for infiltrating a loose orseparated porous material absent the use of a container mold (i.e., itis generally believed that the particulate material would typicallydisassociate or float apart when placed in a molten metal). Stillfurther, if it was desired to infiltrate a particulate material orloosely formed preform, precautions should be taken so that theinfiltrating metal does not displace at least portions of theparticulate or preform resulting in a non-homogeneous microstructure.

Accordingly, there has been a long felt need for a simple and reliableprocess to produce shaped metal matrix composites which does not relyupon the use of applied pressure or vacuum (whether externally appliedor internally created), or damaging wetting agents to create a metalmatrix embedding another material such as a ceramic material. Moreover,there has been a long felt need to minimize the amount of finalmachining operations needed to produce a metal matrix composite body.The present invention satisfies these needs by providing a spontaneousinfiltration mechanism for infiltrating a material (e.g., a ceramicmaterial), which can be formed into a preform, with molten matrix metal(e.g., aluminum) in the presence of an infiltrating atmosphere (e.g.,nitrogen) under normal atmospheric pressures so long as an infiltrationenhancer is present at least at some point during the process.

DESCRIPTION OF COMMONLY OWNED U.S. PATENT APPLICATION

The subject matter of this application is related to that of severalother copending and co-owned patent applications. Particularly, theseother copending patent applications describe novel methods for makingmetal matrix composite materials (hereinafter sometimes referred to as"Commonly Owned Metal Matrix Patent Applications").

A novel method of making a metal matrix composite material is disclosedin Commonly Owned U.S. Pat. No. 4,828,008, which issued on May 9, 1989,from U.S. patent application Ser. No. 07/049,171, filed May 13, 1987, inthe names of White et al., and entitled "Metal Matrix Composites".According to the method of the White et al. invention, a metal matrixcomposite is produced by infiltrating a permeable mass of fillermaterial (e.g., a ceramic or a ceramic-coated material) with moltenaluminum containing at least about 1 percent by weight magnesium, andpreferably at least about 3 percent by weight magnesium. Infiltrationoccurs spontaneously without the application of external pressure orvacuum. A supply of the molten metal alloy is contacted with the mass offiller material at a temperature of at least about 675° C. in thepresence of a gas comprising from about 10 to 100 percent, andpreferably at least about 50 percent, nitrogen by volume, and aremainder of the gas, if any, being a nonoxidizing gas, e.g., argon.Under these conditions, the molten aluminum alloy infiltrates theceramic mass under normal atmospheric pressures to form an aluminum (oraluminum alloy) matrix composite. When the desired amount of fillermaterial has been infiltrated with the molten aluminum alloy, thetemperature is lowered to solidify the alloy, thereby forming a solidmetal matrix structure that embeds the reinforcing filler material.Usually, and preferably, the supply of molten alloy delivered will besufficient to permit the infiltration to proceed essentially to theboundaries of the mass of filler material. The amount of filler materialin the aluminum matrix composites produced according to the White et al.invention may be exceedingly high. In this respect, filler to alloyvolumetric ratios of greater than 1:1 may be achieved.

Under the process conditions in the aforesaid White et al. invention,aluminum nitride can form as a discontinuous phase dispersed throughoutthe aluminum matrix. The amount of nitride in the aluminum matrix mayvary depending on such factors as temperature, alloy composition, gascomposition and filler material. Thus, by controlling one or more suchfactors in the system, it is possible to tailor certain properties ofthe composite. For some end use applications, however, it may bedesirable that the composite contain little or substantially no aluminumnitride.

It has been observed that higher temperatures favor infiltration butrender the process more conducive to nitride formation. The White et.al. invention allows the choice of a balance between infiltrationkinetics and nitride formation.

An example of suitable barrier means for use with metal matrix compositeformation is described in Commonly Owned U.S. Pat. No. 4,935,055, whichissued on Jun. 19, 1990, from U.S. patent application Ser. No.07/141,642, filed Jan. 7, 1988, in the names of Michael K. Aghajanian etal., and entitled "Method of Making Metal Matrix Composite with the Useof a Barrier". According to the method of this Aghajanian et al.invention, a barrier means (e.g., particulate titanium diboride or agraphite material such as a flexible graphite tape product sold by UnionCarbide under the tradename Grafoil®) is disposed on a defined surfaceboundary of a filler material and matrix alloy infiltrates up to theboundary defined by the barrier means. The barrier means is used toinhibit, prevent, or terminate infiltration of the molten alloy, therebyproviding net, or near net, shapes in the resultant metal matrixcomposite. Accordingly, the formed metal matrix composite bodies have anouter shape which substantially corresponds to the inner shape of thebarrier means.

The method of U.S. Pat. No. 4,828,008 was improved upon by CommonlyOwned and Copending U.S. patent application Ser. No. 07/517,541, filedApr. 24, 1990, which is a continuation application of U.S. patentapplication Ser. No. 07/168,284, filed Mar. 15, 1988, in the names ofMichael K. Aghajanian and Marc S. Newkirk and entitled "Metal MatrixComposites and Techniques for Making the Same." In accordance with themethods disclosed in this U.S. patent application, a matrix metal alloyis present as a first source of metal and as a reservoir of matrix metalalloy which communicates with the first source of molten metal due to,for example, gravity flow. Particularly, under the conditions describedin this patent application, the first source of molten matrix alloybegins to infiltrate the mass of filler material under normalatmospheric pressures and thus begins the formation of a metal matrixcomposite. The first source of molten matrix metal alloy is consumedduring its infiltration into the mass of filler material and, ifdesired, can be replenished, preferably by a continuous means, from thereservoir of molten matrix metal as the spontaneous infiltrationcontinues. When a desired amount of permeable filler has beenspontaneously infiltrated by the molten matrix alloy, the temperature islowered to solidify the alloy, thereby forming a solid metal matrixstructure that embeds the reinforcing filler material. It should beunderstood that the use of a reservoir of metal is simply one embodimentof the invention described in this patent application and it is notnecessary to combine the reservoir embodiment with each of the alternateembodiments of the invention disclosed therein, some of which could alsobe beneficial to use in combination with the present invention.

The reservoir of metal can be present in an amount such that it providesfor a sufficient amount of metal to infiltrate the permeable mass offiller material to a predetermined extent. Alternatively, an optionalbarrier means can contact the permeable mass of filler on at least oneside thereof to define a surface boundary.

Moreover, while the supply of molten matrix alloy delivered should be atleast sufficient to permit spontaneous infiltration to proceedessentially to the boundaries (e.g., barriers) of the permeable mass offiller material, the amount of alloy present in the reservoir couldexceed such sufficient amount so that not only will there be asufficient amount of alloy for complete infiltration, but excess moltenmetal alloy could remain and be attached to the metal matrix compositebody. Thus, when excess molten alloy is present, the resulting body willbe a complex composite body (e.g., a macrocomposite), wherein aninfiltrated ceramic body having a metal matrix therein will be directlybonded to excess metal remaining in the reservoir.

Each of the above-discussed Commonly Owned Metal Matrix Patents andPatent Applications describes methods for the production of metal matrixcomposite bodies and novel metal matrix composite bodies which areproduced therefrom. The entire disclosures of all of the foregoingCommonly Owned Metal Matrix Patents and Patent Applications areexpressly incorporated herein by reference.

SUMMARY OF THE INVENTION

A metal matrix composite body is produced by spontaneously infiltratinga permeable mass of filler material with matrix metal. Specifically, aninfiltration enhancer and/or an infiltration enhancer precursor and/oran infiltrating atmosphere are in communication with the fillermaterial, at least at some point during the process, which permitsmolten matrix metal to spontaneously infiltrate the filler material.After substantially complete infiltration has been obtained, additionalmatrix metal (sometimes also referred to herein as a second matrixmetal), whether of the same, similar or different composition from thematrix metal which has already infiltrated the filler material, isthereafter added to the infiltrated filler material (e.g., in apreferred embodiment is physically admixed with the infiltrated fillermaterial) to result in a suspension of filler material and matrix metal.Such a suspension has a lower loading of filler material relative to thematrix metal. In an alternative embodiment, an excess of matrix metal isprovided, which remains as molten uninfiltrated metal, after spontaneousinfiltration is complete. The excess of matrix metal is thereafterstirred or mixed into the infiltrated filler material to form asuspension of filler material and matrix metal having a lower particleloading than the originally spontaneously infiltrated filler material.

In an alternative embodiment, the spontaneously infiltrated metal matrixcomposite can be allowed to cool after infiltration is complete to forma highly loaded metal matrix composite. The composite can thereafter bereheated to at least its liquidus temperature and a second or additionalmatrix metal can be admixed therewith.

In a preferred embodiment, a precursor to an infiltration enhancer maybe supplied to at least one, or both, of the matrix metal and the fillermaterial. The combination of filler material, matrix metal, supply ofinfiltration enhancer precursor and infiltrating atmosphere causes thematrix metal to spontaneously infiltrate the filler material.

Moreover, rather than supplying an infiltration enhancer precursor, aninfiltration enhancer may be supplied directly to at least one of thefiller material, and/or matrix metal, and/or infiltrating atmosphere.Ultimately, at least during the spontaneous infiltration, theinfiltration enhancer should be located in at least a portion of thefiller material.

The physical admixing of additional matrix metal can be achieved bymechanical stirring means, ultrasonic stirring means, vibrating means,by hand stirring, or by any other suitable means of mixing theinfiltrated filler material with additional matrix metal.

Moreover, as stated previously herein, the second or additional matrixmetal can have a composition which is smaller to or quite different fromthe matrix metal which infiltrated the filler material. In the case ofutilizing a different second matrix metal, it would be preferable forthe first matrix metal which infiltrated the filler material to be atleast partially miscible with the second matrix metal to result in analloying of the first and second matrix metals and/or the formation ofintermetallics of the first and second matrix metals. When the secondmatrix metal is substantially similar to or the same as the first matrixmetal which infiltrated the filler material, the two matrix metals arelikely to mix quite readily.

After a desirable mixing of the first and second matrix metals has beenachieved, the suspension of infiltrated filler material and first andsecond matrix metals can be allowed to cool, in situ, in the mixingchamber, if desired. The cooled mixture can thereafter be reheated to atemperature at or above the liquidus temperature of the matrix metal inthe suspension and thereafter poured into a desired mold. Alternatively,if the mixture is contained within a mold which corresponded to adesired final shape, the mixture can simply be allowed to cool andthereafter be removed from the mold. Still further, the mixture can bemaintained in a molten state and poured into a desired mold, whicheither corresponds to a final metal matrix composite body to be producedor corresponds to some intermediate shape (e.g., an ingot) forsubsequent processing.

The resultant metal matrix composite body containing both first andsecond matrix metals will have a lower volume fraction of fillermaterial relative to a metal matrix composite body which does notcontain a second matrix metal therein. Accordingly, the presentinvention provides a method for producing metal matrix composite bodieshaving lower volume fractions of filler material. Such lower volumefractions of filler material typically can not be effectively achievedby spontaneously infiltrating a very porous filler material because themaximum amount of porosity that a filler material can exhibit is limiteddue to such considerations as minimal packing density, preform strength,etc.

It is noted that this application discusses primarily aluminum matrixmetals which, at some point during the formation of the metal matrixcomposite body, are contacted with magnesium, which functions as theinfiltration enhancer precursor, in the presence of nitrogen, whichfunctions as the infiltrating atmosphere. Thus, the matrixmetal/infiltration enhancer precursor//infiltrating atmosphere system ofaluminum/magnesium/nitrogen exhibits spontaneous infiltration. However,other matrix metal/infiltration enhancer precursor/infiltratingatmosphere systems may also behave in a manner similar to the systemaluminum/magnesium/nitrogen. For example, similar spontaneousinfiltration behavior has been observed in thealuminum/strontium/nitrogen system; the aluminum/zinc/oxygen system; andthe aluminum/calcium/nitrogen system. Accordingly, even though thealuminum/magnesium/nitrogen system is discussed primarily herein, itshould be understood that other matrix metal/infiltration enhancerprecursor/infiltrating atmosphere systems may behave in a similarmanner.

When the matrix metal comprises an aluminum alloy, the aluminum alloy iscontacted with a filler material (e.g., alumina or silicon carbideparticles). In one embodiment of the invention, the filler material hasadmixed therewith, or at some point during the process is exposed to,magnesium, as an infiltration enhancer precursor. The aluminum alloyand/or the filler material at some point during the process, and in thispreferred embodiment during substantially all of the process, areexposed to a nitrogen atmosphere. In an alternative embodiment, thefiller material, and/or the aluminum alloy, and/or the nitrogeninfiltrating atmosphere, contain magnesium nitride, as an infiltrationenhancer. In either embodiment, the filler material will bespontaneously infiltrated by the matrix metal and the extent or rate ofspontaneous infiltration and formation of metal matrix will vary with agiven set of process conditions including, for example, theconcentration of magnesium provided to the system (e.g., in the aluminumalloy and/or in the filler material and/or in the infiltratingatmosphere), the size and/or composition of the particles comprising thefiller material, the concentration of nitrogen in the infiltratingatmosphere, the time permitted for infiltration, and/or the temperatureat which infiltration occurs. Spontaneous infiltration typically occursto an extent sufficient to embed substantially completely the fillermaterial.

DEFINITIONS

"Aluminum", as used herein, means and includes essentially pure metal(e.g., a relatively pure, commercially available unalloyed aluminum) orother grades of metal and metal alloys such as the commerciallyavailable metals having impurities and/or alloying constituents such asiron, silicon, copper, magnesium, manganese, chromium, zinc, etc.,therein. An aluminum alloy for purposes of this definition is an alloyor intermetallic compound in which aluminum is the major constituent."Balance Non-Oxidizing Gas", as used herein, means that any gas presentin addition to the primary gas comprising the infiltrating atmosphere iseither an inert gas or a reducing gas which is substantiallynon-reactive with the matrix metal under the process conditions. Anyoxidizing gas which may be present as an impurity in the gas(es) usedshould be insufficient to oxidize the matrix metal to any substantialextent under the process conditions.

"Barrier" or "barrier means", as used herein, means any suitable meanswhich interferes, inhibits, prevents or terminates the migration,movement, or the like, of molten matrix metal beyond a surface boundaryof a permeable mass of filler material or preform, where such surfaceboundary is defined by said barrier means. Suitable barrier means may beany such material, compound, element, composition, or the like, which,under the process conditions, maintains some integrity and is notsubstantially volatile (i.e., the barrier material does not volatilizeto such an extent that it is rendered non-functional as a barrier).

Further, suitable "barrier means" includes materials which aresubstantially non-wettable by the migrating molten matrix metal underthe process conditions employed. A barrier of this type appears toexhibit substantially little or no affinity for the molten matrix metal,and movement beyond the defined surface boundary of the mass of fillermaterial or preform is prevented or inhibited by the barrier means. Thebarrier reduces any final machining or grinding that may be required anddefines at least a portion of the surface of the resulting metal matrixcomposite product. The barrier may in certain cases be permeable orporous, or rendered permeable by, for example, drilling holes orpuncturing the barrier, to permit gas to contact the molten matrixmetal. "Carcass" or "Carcass of Matrix Metal", as used herein, refers tosay of the original body of matrix metal remaining which has not beenconsumed during formation of the metal matrix composite body, andtypically, if allowed to cool, remains in at least partial contact withthe metal matrix composite body which has been formed. It should beunderstood that the carcass may also include a second or foreign metaltherein. "Filler", as used herein, is intended to include either singleconstituents or mixtures of constituents which are substantiallynon-reactive with and/or of limited solubility in the matrix metal andmay be single or multi-phase. Fillers may be provided in a wide varietyof forms, such as powders, flakes, platelets, microspheres, whiskers,bubbles, etc., and may be either dense or porous. "Filler" may alsoinclude ceramic fillers, such as alumina or silicon carbide as fibers,chopped fibers, particulates, whiskers, bubbles, spheres, fiber mats, orthe like, and ceramic-coated fillers such as carbon fibers coated withalumina or silicon carbide to protect the carbon from attack, forexample, by a molten aluminum parent metal. Fillers may also includemetals. "Highly Loaded Metal Matrix Composite", as used herein, means ametal matrix composite material which has been formed by the spontaneousinfiltration of a matrix metal into a filler material and which fillermaterial has not had any substantial amount of second or additionalmatrix metal added thereto. "Infiltrating Atmosphere", as used herein,means that atmosphere which is present which interacts with the matrixmetal and/or preform (or infiltration enhancer and permits or enhancesspontaneous infiltration of the matrix metal to occur.

"Infiltration Enhancer", as used herein, means a material which promotesor assists in the spontaneous infiltration of a matrix metal into afiller material or preform. An infiltration enhancer may be formed from,for example, a reaction of an infiltration enhancer precursor with aninfiltrating atmosphere to form (1) a gaseous species and/or (2) areaction product of the infiltration enhancer precursor and theinfiltrating atmosphere and/or (3) a reaction product of theinfiltration enhancer precursor and the filler material or preform.Moreover, the infiltration enhancer may be supplied directly to at leastone of the preform, and/or matrix metal, and/or infiltrating atmosphereand function in a substantially similar manner to an infiltrationenhancer which has formed as a reaction between an infiltration enhancerprecursor and another species. Ultimately, at least during thespontaneous infiltration, the infiltration enhancer should be located inat least a portion of the filler material or preform to achievespontaneous infiltration. "Infiltration Enhancer Precursor" or"Precursor to the Infiltration Enhancer", as used herein, means amaterial which when used in combination with the matrix metal, preformand/or infiltrating atmosphere forms an infiltration enhancer whichinduces or assists the matrix metal to spontaneously infiltrate thefiller material or preform. Without wishing to be bound by anyparticular theory or explanation, it appears as though it may benecessary for the precursor to the infiltration enhancer to be capableof being positioned, located or transportable to a location whichpermits the infiltration enhancer precursor to interact with theinfiltrating atmosphere and/or the preform or filler material and/ormetal. For example, in some matrix metal/infiltration enhancerprecursor/infiltrating atmosphere systems, it is desirable for theinfiltration enhancer precursor to volatilize at, near, or in somecases, even somewhat above the temperature at which the matrix metalbecomes molten. Such volatization may lead to: (1) a reaction of theinfiltration enhancer precursor with the infiltrating atmosphere to forma gaseous species which enhances wetting of the filler material orpreform by the matrix metal; and/or (2) a reaction of the infiltrationenhancer precursor with the infiltrating atmosphere to form a solid,liquid or gaseous infiltration enhancer in at least a portion of thefiller material or preform which enhances wetting; and/or (3) a reactionof the infiltration enhancer precursor within the filler material orpreform which forms a solid, liquid or gaseous infiltration enhancer inat least an portion of the filler material or preform which enhanceswetting.

"Low Particle Loading" or "Lower Volume Fraction of Filler Material", asused herein, means that the amount of matrix metal relative to fillermaterial has been increased relative to a filler material which isspontaneously infiltrated without having an additional or second matrixalloy added thereto. "Matrix Metal" or "Matrix Metal Alloy", as usedherein , means that metal which is utilized to form a metal matrixcomposite (e.g., before infiltration) and/or that metal which isintermingled with a filler material to form a metal matrix compositebody (e.g., after infiltration). When a specified metal is mentioned asthe matrix metal, it should be understood that such matrix metalincludes that metal as an essentially pure metal, a commerciallyavailable metal having impurities and/or alloying constituents therein,an intermetallic compound or an alloy in which that metal is the majoror predominant constituent.

"Matrix Metal/Infiltration Enhancer Precursor/Infiltrating AtmosphereSystem" or "Spontaneous System", as used herein, refers to thatcombination of materials whichs exhibit spontaneous infiltration into apreform or filler material. It should be understood that whenever a "/"appears between an exemplary matrix metal, infiltration enhancerprecursor and infiltrating atmosphere, the "/" is used to designate asystem or combination of materials which, when combined in a particularmanner, exhibits spontaneous infiltration into a preform or fillermaterial.

"Metal Matrix Composite"]or "MMC", as used herein, means a materialcomprising a two- or three-dimensionally interconnected alloy or matrixmetal which has embedded a preform or filler material. The matrix metalmay include various alloying elements to provide specifically desiredmechanical and physical properties in the resulting composite.

A Metal "Different" from the Matrix Metal means a metal which does notcontain, as a primary constituent, the same metal as the matrix metal(e.g., if the primary constituent of the matrix metal is aluminum, the"different" metal could have a primary constituent of, for example,nickel). "Nonreactive Vessel for Housing Matrix Metal" means any vesselwhich can house or contain a filler material (or preform) and/or moltenmatrix metal under the process conditions and not react with the matrixand/or the infiltrating atmosphere and/or infiltration enhancerprecursor and/or a filler material (or preform) in a manner which wouldbe significantly detrimental to the spontaneous infiltration mechanism."Preform" or "Permeable Preform", as used herein, means a porous mass offiller or filler material which is manufactured with at least onesurface boundary which essentially defines a boundary for infiltratingmatrix metal, such mass retaining sufficient shape integrity and greenstrength to provide dimensional fidelity prior to being infiltrated bythe matrix metal. The mass should be sufficiently porous to accommodatespontaneous infiltration of the matrix metal thereinto. A preformtypically comprises a bonded array or arrangement of filler, eitherhomogeneous or heterogeneous, and may be comprised of any suitablematerial (e.g., ceramic and/or metal particulates, powders, fibers,whiskers, etc., and any combination thereof). A preform may exist eithersingularly or as an assemblage. "Reservoir", as used herein, means aseparate body of matrix metal positioned relative to a mass of filler ora preform so that, when the metal is molten, it may flow to replenish,or in some cases to initially provide and subsequently replenish, thatportion, segment or source of matrix metal which is in contact with thefiller or preform. "Second Matrix Metal" or "Additional Matrix Metal",as used herein, means that metal which remains or which is added afterspontaneous infiltration of the filler material has been completed orsubstantially completed, and which is admixed with the infiltratedfiller material to form a suspension of infiltrated filler material andfirst and second (or additional) matrix metals, thereby forming a lowervolume fraction of filler material, such second or additional matrixmetal having a composition which either is exactly the same as, similarto or substantially different from the matrix metal which has previouslyspontaneously infiltrated the filler material. "SpontaneousInfiltration", as used herein, means the infiltration of matrix metalinto the permeable mass of filler or preform occurs without requirementfor the application of pressure or vacuum (whether externally applied orinternally created). "Suspension of Filler Material and Matrix Metal" or"Suspension", or "Metal Matrix Composite Suspension", as used herein,means the mixture of second or additional matrix metal and fillermaterial which has been spontaneously infiltrated by a first matrixmetal.

BRIEF DESCRIPTION OF THE FIGURES

The following figures are provided to assist in understanding theinvention, but are not intended to limit the scope of the invention.Similar reference numerals have been used wherever possible in each ofthe Figures to denote like components, wherein:

FIG. 1a is a schematic cross-sectional view of a lay-up in accordancewith the present invention, illustrating a partially infiltratedcomposite with excess matrix metal;

FIG. 1b is a schematic cross-sectional view illustrating the dispersionof an infiltrated composite and excess matrix metal;

FIG. 1c is a schematic cross-sectional view of the dispersed infiltratedcomposite before further processing;

FIG. 1d is a schematic cross-sectional view illustrating the pourabilityof the dispersed composite;

FIG. 2 is a schematic cross-sectional view of the lay-up of Examples1-4;

FIG. 3 is a cross-sectional schematic view of a lay-up used to fabricatethe highly loaded metal matrix composite body of Example 5;

FIG. 4a is a cross-sectional schematic view of which shows theintroduction of the highly loaded metal matrix composite material intothe melt comprising the second matrix metal contained within a crucibleand the crushing of any loosely bound filler material from the highlyloaded metal matrix composite;

FIG. 4b is a cross-sectional schematic view which shows the introductionof a stirring means into the crucible containing molten first and secondmatrix metals and the crushed filler material of the highly loaded metalmatrix composite material;

FIG. 4c is a cross-sectional schematic view which shows a formed metalmatrix composite suspension;

FIG. 4d is a cross-sectional schematic view which shows the pouring ofthe metal matrix composite suspension from the crucible to form a castmetal matrix composite material;

FIG. 5 is a cross-sectional schematic view of a lay-up used to fabricatethe highly loaded metal matrix composite of Example 6;

FIG. 6 shows the geometry of the pin-loaded tensile bar used to measurethe tensile strength of the cast metal matrix composite body of Example6;

FIG. 7 is a cross-sectional schematic view of a lay-up used to fabricatethe highly loaded metal matrix composite of Example 7;

FIGS. 8a-8e show schematically the process sequence for fabricating themetal matrix composite body of Example 7;

FIG. 9 is a photograph of a golf club head comprising a metal body and acast metal matrix composite insert;

FIG. 10 is a cross-sectional schematic view of a lay-up used tofabricate the highly loaded metal matrix composite of Example 8;

FIG. 11 is a photograph of the cast metal matrix composite tensile barand its attached gates and risers whose fabrication is described inExample 8;

FIGS. 12a and 12b are optical photomicrographs taken at approximately 50× and 400× magnifications, respectively, of a polished cross section ofthe cast metal matrix composite material formed in Example 8;

FIG. 13 is a photograph of the cast fluidity spiral which is describedin Example 9; and

FIGS. 14a and 14b are optical photomicrographs taken at approximately50× and 400× magnifications, respectively, of a polished cross sectionof the cast metal matrix composite material formed in Example 9.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

Although high particle loadings (of the order of 40 to 60 volumepercent) are obtainable from spontaneous infiltration techniques asdisclosed, for example, in commonly owned U.S. Pat. No. 4,828,008, lowerparticle loadings (of the order of 1 to 40 volume percent) are moredifficult, in some cases, to obtain using only such techniques.Specifically, to obtain lower particle loadings with such techniques,preforms or filler material having very high porosity may be required.However, the porosity ultimately obtainable with such filler materialsor preforms is limited, such porosity being a function of theperpendicular filler material employed and the size or granularity ofthe particles used in the preform.

In accordance with the present invention, spontaneous infiltrationtechniques are utilized to obtain the advantageous properties heretoforeassociated with spontaneously infiltrated metal matrix composites, yetlower particle loadings are obtainable. A metal matrix composite body isformed by first spontaneously infiltrating a filler material with afirst matrix metal in an infiltrating atmosphere and thereafter addingadditional or second matrix metal to the infiltrated filler material toresult in a suspension of lower volume fraction of filler material andmatrix metal. Furthermore, as discussed in detail below, the addition ofthe additional or second matrix metal enables the process to be tailoredto provide a metal matrix of the first matrix metal (i.e., where thefirst and second matrix metals are the same) or an intermetallic oralloy of the first and second matrix metals (i.e., where the first andsecond matrix metals are different).

As a first step in the process for obtaining low particle loading metalmatrix composites, spontaneous infiltration of a filler material orpreform is initiated.

With reference to the figures, FIG. 1a illustrates a lay-up (10) whichcould be used in accordance with the present invention. Specifically, afiller material (or preform) (1) is provided in a mold or container (2),which is substantially non-reactive with the components. A matrix metal(3) is provided, and is heated above its melting point under conditionswhich enable spontaneous infiltration to occur, as discussed in moredetail below. As the matrix metal begins to spontaneously infiltrate thefiller material or preform, a metal matrix composite (4) is formed(e.g., if the matrix metal was permitted to cool, at least the portion(4) would comprise a metal matrix composite).

In a first preferred embodiment of the invention, an excess of matrixmetal (3) is provided, such that upon completion of spontaneousinfiltration, a carcass of uninfiltrated matrix metal remains.

As illustrated in FIG. 1b, the matrix metal composite, while stillmolten, is admixed with excess matrix metal via a stirrer (5), such thatthe infiltrated filler material is dispersed into the additional matrixmetal to form a suspension. Stirrer (5) can be any conventional stirringapparatus, including mechanical stirring means, ultrasonic stirringmeans or hand stirring. Stirring is continued for 1 to 15 minutes, andpreferably for 10 to 15 minutes, or until a homogeneous, fully dispersedmixture (7) is obtained, as illustrated by FIG. 1c.

Stirring should preferably be undertaken at spontaneous infiltrationprocess temperatures (discussed below) to avoid hardening of thecomposite before dispersion of the mixture is complete. Such stirringcould be effected, for example, via overhead stirring means provided inthe furnace. Alternatively, if stirring is not performed at processtemperatures, procedures should be undertaken to avoid prematurecooling, including the use of heated stirring apparatus andwell-insulated containment vessels, etc.

After complete dispersion is achieved, the dispersed mixture can bepoured, as illustrated in FIG. 1d, into a mold to form a body having alower particle loading than is otherwise obtainable via spontaneousinfiltration. Any conventional mold can be used, such as investmentshell molds, split shell molds, multiple piece molds, reusable molds,and the like. The molds, preferably, are heated to delay cooling of thedispersed composite to maximize pour times and workability of the poureddispersed composite. Alternatively, room temperature molds or cooledmolds, e.g., a copper chill plate, can be utilized if quicker cooling isdesired in a particular application.

In an alternative embodiment, the container in which the composite isdispersed corresponds to the ultimate desired shape of the body to beformed. Accordingly, rather than pouring the suspension of fillermaterial and matrix metal, it is allowed to cool in the container, suchthat the container performs the function of the mold. Alternatively, thesuspension can be allowed to cool, and can thereafter be reheated aboveits melting point and poured or molded for further processing orforming. Moreover, the suspension can be poured into a mold to form anintermediate body, e.g., an ingot, which can thereafter be used as aprecursor to further processing.

The resulting composite from the above embodiments exhibits the highlydesirable properties associated with other spontaneously infiltratedcomposites. Moreover, lower particle loadings are obtainable, e.g., ofthe order of 5 to 40 volume percent, using the dispersion methods of thepresent invention.

In a further embodiment of the present invention, an excess of matrixmetal is not used as in FIG. 1a. Instead, a bed of filler material or apreform is spontaneously infiltrated and allowed to cool to form ahighly loaded metal matrix composite. The highly loaded metal matrixcomposite is thereafter reheated and additional matrix metal isdispersed therein in accordance with the procedures discussed above tocreate a suspension of filler material and matrix metal, said suspensionbeing capable of forming a low particle loading metal matrix composite(e.g., by a casting method). Alternatively, the additional matrix metalcan be added while the matrix metal in the infiltrated composite isstill in its liquid state.

The additional or second matrix metal in all of the above embodimentscan have a composition which is the same as, similar to or quitedifferent from the matrix metal which spontaneously infiltrates thefiller or preform. Through the use of different first and second matrixmetals, the resultant three dimensionally interconnected metal matrix ofthe metal matrix composite can be varied to provide any of a variety ofalloys or intermetallics to suit a particular application. As a result,desired chemical, electrical, mechanical and other properties can betailored to suit a particular application. The second matrix metal is,preferably, a metal which is miscible with the first matrix metal.

The second matrix metal can be introduced in many different ways. Withreference to FIG. 1a, matrix metal (3) could be a multi-phase moltenalloy having stratified layers comprised at its surface adjacent to theinterface with the filler of a first matrix metal, but having at itsupper end a second matrix metal. The first matrix metal can be, forexample, rich in infiltration enhancer and/or infiltration enhancerprecursor and/or secondary alloys which promote infiltration. After thefirst matrix metal spontaneously infiltrates, the second or additionalmatrix metal can be admixed to the suspension in accordance with FIG.1b.

Alternatively, the second or additional matrix metal can be poured in oradded in its solid form and liquefied, after spontaneous infiltrationhas occurred. Moreover, as discussed above, a metal matrix composite canbe formed and cooled and, in a subsequent processing step, the compositecan be reheated and the second or additional matrix metal can bedispersed into the suspension.

In order to effect spontaneous infiltration of the matrix metal into thepreform, an infiltration enhancer should be provided to the spontaneoussystem. An infiltration enhancer could be formed from an infiltrationenhancer precursor which could be provided (1) in the matrix metal;and/or (2) in the preform; and/or (3) from the infiltrating atmosphere;and/or (4) from an external source into the spontaneous system.Moreover, rather than supplying an infiltration enhancer precursor, aninfiltration enhancer may be supplied directly to at least one of thepreform, and/or matrix metal, and/or infiltrating atmosphere.Ultimately, at least during the spontaneous infiltration, theinfiltration enhancer should be located in at least a portion of thefiller material.

In a preferred embodiment it is possible that the infiltration enhancerprecursor can be at least partially reacted with the infiltratingatmosphere such that infiltration enhancer can be formed in at least aportion of the filler prior to or substantially contiguous withcontacting the filler material with the matrix metal (e.g., if magnesiumwas the infiltration enhancer precursor and nitrogen was theinfiltrating atmosphere, the infiltration enhancer could be magnesiumnitride which would be located in at least a portion of the fillermaterial).

An example of a matrix metal/infiltration enhancerprecursor/infiltrating atmosphere system is thealuminum/magnesium/nitrogen system. Specifically, an aluminum matrixmetal can be contained within a suitable refractory vessel which, underthe process conditions, does not react with the aluminum matrix metaland/or the filler material when the aluminum is made molten. A fillermaterial can then be contacted with the molten aluminum matrix metal.Under the process conditions, the aluminum matrix metal is induced toinfiltrate the filler material spontaneously.

Under the conditions employed in the method of the present invention, inthe case of an aluminum/magnesium/nitrogen spontaneous infiltrationsystem, the filler material should be sufficiently permeable to permitthe nitrogen-containing gas to penetrate or permeate the molten materialat some point during the process and/or contact the molten matrix metal.Moreover, the permeable preform can accommodate infiltration of themolten matrix metal, thereby causing the nitrogen-permeated preform tobe infiltrated spontaneously with molten matrix metal to form a metalmatrix composite body and/or cause the nitrogen to react with aninfiltration enhancer precursor to form infiltration enhancer in thepreform and thereby resulting in spontaneous infiltration. The extent ofspontaneous infiltration and formation of the metal matrix compositewill vary with a given set of process conditions, including magnesium ormagnesium nitride content of the aluminum alloy, magnesium or magnesiumnitride content of the filler material, amount of magnesium or magnesiumnitride in the filler material, the presence of additional alloyingelements (e.g., silicon, iron, copper, manganese, chromium, zinc, andthe like), average size of the materials comprising the filler material(e.g., particle diameter) surface condition and type of filler material,nitrogen concentration of the infiltrating atmosphere, time permittedfor infiltration and temperature at which infiltration occurs. Forexample, for infiltration of the molten aluminum matrix metal to occurspontaneously, the aluminum can be alloyed with at least about 1 percentby weight, and preferably at least about 3 percent by weight, magnesium(which functions as the infiltration enhancer precursor), based on alloyweight. Auxiliary alloying elements, as discussed above, may also beincluded in the matrix metal to tailor specific properties thereof.(Additionally, the auxiliary alloying elements may affect the minimumamount of magnesium required in the matrix aluminum metal to result inspontaneous infiltration of the filler material or preform). Loss ofmagnesium from the spontaneous system due to, for example,volatilization should not occur to such an extent that no magnesium waspresent to form infiltration enhancer. Thus, it is desirable to utilizea sufficient amount of initial alloying elements to assure thatspontaneous infiltration will not be adversely affected byvolatilization. Still further, the presence of magnesium in both of thepreform and matrix metal or the preform alone may result in a reductionin the required amount of magnesium to achieve spontaneous infiltration(discussed in greater detail later herein).

The volume percent of nitrogen in the nitrogen atmosphere also affectsformation rates of the metal matrix composite body. Specifically, ifless than about 10 volume percent of nitrogen is present in theinfiltrating atmosphere, very slow or little spontaneous infiltrationwill occur. It has been discovered that it is preferable for at leastabout 50 volume percent of nitrogen to be present in the atmosphere,thereby resulting in, for example, shorter infiltration times due to amuch more rapid rate of infiltration. The infiltrating atmosphere (e.g.,a nitrogen-containing gas) can be supplied directly to the fillermaterial or preform and/or matrix metal, or it may be produced or resultfrom a decomposition of a material.

The minimum magnesium content required for molten matrix metal toinfiltrate a filler material or preform depends on one or more variablessuch as the processing temperature, time, the presence of auxiliaryalloying elements such as silicon or zinc, the nature of the fillermaterial, the location of the magnesium in one or more components of thespontaneous system, the nitrogen content of the atmosphere, and the rateat which the nitrogen atmosphere flows. Lower temperatures or shorterheating times can be used to obtain complete infiltration as themagnesium content of the alloy and/or preform is increased. Also, for agiven magnesium content, the addition of certain auxiliary alloyingelements such as zinc permits the use of lower temperatures. Forexample, a magnesium content of the matrix metal at the lower end of theoperable range, e.g., from about 1 to 3 weight percent, may be used inconjunction with at least one of the following: an above-minimumprocessing temperature, a high nitrogen concentration, or one or moreauxiliary alloying elements. When no magnesium is added to the preform,alloys containing from about 3 to 5 weight percent magnesium arepreferred on the basis of their general utility over a wide variety ofprocess conditions, with at least about 5 percent being preferred whenlower temperatures and shorter times are employed. Magnesium contents inexcess of about 10 percent by weight of the aluminum alloy may beemployed to moderate the temperature conditions required forinfiltration. The magnesium content may be reduced when used inconjunction with an auxiliary alloying element, but these elements servean auxiliary function only and are used together with at least theabove-specified minimum amount of magnesium. For example, there wassubstantially no infiltration of nominally pure aluminum alloyed onlywith 10 percent silicon at 1000° C. into a bedding of 500 mesh, 39crystolon (99 percent pure silicon carbide from Norton Co.). However, inthe presence of magnesium, silicon has been found to promote theinfiltration process. As a further example, the amount of magnesiumvaries if it is supplied exclusively to the preform or filler material.It has been discovered that spontaneous infiltration will occur with alesser weight percent of magnesium supplied to the spontaneous systemwhen at least some of the total amount of magnesium supplied is placedin the preform or filler material. It may be desirable for a lesseramount of magnesium to be provided in order to prevent the formation ofundesirable intermetallics in the metal matrix composite body. In thecase of a silicon carbide preform, it has been discovered that when thepreform is contacted with an aluminum matrix metal, the preformcontaining at least about 1% by weight magnesium and being in thepresence of a substantially pure nitrogen atmosphere, the matrix metalspontaneously infiltrates the preform. In the case of an aluminapreform, the amount of magnesium required to achieve acceptablespontaneous infiltration is slightly higher. Specifically, it has beenfound that when an alumina preform, when contacted with a similaraluminum matrix metal, at about the same temperature as the aluminumthat infiltrated into the silicon carbide preform, and in the presenceof the same nitrogen atmosphere, at least about 3% by weight magnesiummay be required to achieve similar spontaneous infiltration to thatachieved in the silicon carbide preform discussed immediately above.

It is also noted that it is possible to supply to the spontaneous systeminfiltration enhancer precursor and/or infiltration enhancer on asurface of the alloy and/or on a surface of the preform or fillermaterial and/or within the preform or filler material prior toinfiltrating the matrix metal into the filler material or preform (i.e.,it may not be necessary for the supplied infiltration enhancer orinfiltration enhancer precursor to be alloyed with the matrix metal, butrather, simply supplied to the spontaneous system). If the magnesium wasapplied to a surface of the matrix metal, it may be preferred that saidsurface should be the surface which is closest to, or preferably incontact with, the permeable mass of filler material or vice versa; orsuch magnesium could be mixed into at least a portion of the preform orfiller material. Still further, it is possible that some combination ofsurface application, alloying and placement of magnesium into at least aportion of the preform could be used. Such combination of applyinginfiltration enhancer(s) and/or infiltration enhancer precursor(s) couldresult in a decrease in the total weight percent of magnesium needed topromote infiltration of the matrix aluminum metal into the preform, aswell as achieving lower temperatures at which infiltration can occur.Moreover, the amount of undesirable intermetallics formed due to thepresence of magnesium could also be minimized.

The use of one or more auxiliary alloying elements and the concentrationof nitrogen in the surrounding gas also affects the extent of nitridingof the matrix metal at a given temperature. For example, auxiliaryalloying elements such as zinc or iron included in the alloy, or placedon a surface of the alloy, may be used to reduce the infiltrationtemperature and thereby decrease the amount of nitride formation,whereas increasing the concentration of nitrogen in the gas may be usedto promote nitride formation.

The concentration of magnesium in the alloy, and/or placed onto asurface of the alloy, and/or combined in the filler material, also tendsto affect the extent of infiltration at a given temperature.Consequently, in some cases where little or no magnesium is contacteddirectly with the filler material, it may be preferred that at leastabout three weight percent magnesium be included in the alloy. Alloycontents of less than this amount, such as one weight percent magnesium,may require higher process temperatures or an auxiliary alloying elementfor infiltration. The temperature required to effect the spontaneousinfiltration process of this invention may be lower (1) when themagnesium content of the alloy alone is increased, e.g. to at leastabout 5 weight percent; and/or (2) when alloying constituents are mixedwith the permeable mass of filler material; and/or (3) when anotherelement such as zinc or iron is present in the aluminum alloy. Thetemperature also may vary with different filler materials. In general,spontaneous and progressive infiltration will occur at a processtemperature of at least about 675° C., and preferably a processtemperature of at least about 750° C.-800° C. Moreover, satisfactorypourability of the resulting suspension, after the second matrix metalhas been dispersed, is achievable at or about 800° C. or greater, andpossibly lower, depending upon the nature of the suspension. Pourabilitydoes not necessarily improve with increased temperatures. Temperaturesgenerally in excess of 1200° C. do not appear to benefit the process,and a particularly useful temperature range has been found to be fromabout 675° C. to about 1200° C. However, as a general rule, thespontaneous infiltration temperature is a temperature which is above themelting point of the matrix metal but below the volatilizationtemperature of the matrix metal. Moreover, the spontaneous infiltrationtemperature should be below the melting point of the filler material.Still further, as temperature is increased, the tendency to form areaction product between the matrix metal and infiltrating atmosphereincreases (e.g., in the case of aluminum matrix metal and a nitrogeninfiltrating atmosphere, aluminum nitride may be formed). Such reactionproduct may be desirable or undesirable based upon the intendedapplication of the metal matrix composite body. Additionally, electricresistance heating is typically used to achieve the infiltratingtemperatures. However, any heating means which can cause the matrixmetal to become molten and does not adversely affect spontaneousinfiltration, is acceptable for use with the invention.

In the present method, for example, a permeable filler material comesinto contact with molten aluminum in the presence of, at least some timeduring the process, a nitrogen-containing gas. The nitrogen-containinggas may be supplied by maintaining a continuous flow of gas into contactwith at least one of the filler material and molten aluminum matrixmetal. Although the flow rate of the nitrogen-containing gas is notcritical, it is preferred that the flow rate be sufficient to compensatefor any nitrogen lost from the atmosphere due to nitride formation inthe alloy matrix, and also to prevent or inhibit the incursion of airwhich can have an oxidizing effect on the molten metal.

The method of forming a metal matrix composite is applicable to a widevariety of filler materials, and the choice of filler materials willdepend on such factors as the matrix alloy, the process conditions, thereactivity of the molten matrix alloy with the filler material, and theproperties sought for the final composite product. For example, whenaluminum is the matrix metal, suitable filler materials include (a)oxides, e.g. alumina; (b) carbides, e.g. silicon carbide; (c) borides,e.g. aluminum dodecaboride, and (d) nitrides, e.g. aluminum nitride. Ifthere is a tendency for the filler material to react with the moltenaluminum matrix metal, this might be accommodated by minimizing theinfiltration time and temperature or by providing a non-reactive coatingon the filler. The filler material may comprise a substrate, such ascarbon or other non-ceramic material, bearing a ceramic coating toprotect the substrate from attack or degradation. Suitable ceramiccoatings include oxides, carbides, borides and nitrides. Ceramics whichare preferred for use in the present method include alumina and siliconcarbide in the form of particles, platelets, whiskers and fibers. Thefibers can be discontinuous (in chopped form) or in the form ofcontinuous filament, such as multifilament tows. Further, the fillermaterial or preform may be homogeneous or heterogeneous. Alumina andsilicon carbide both provide satisfactory suspensions when a second oradditional matrix metal is dispersed therein in accordance with theinvention. As discussed in greater detail in the examples, siliconcarbide has been found to be more pourable then alumina after dispersioninto a suspension.

It also has been discovered that certain filler materials exhibitenhanced infiltration relative to filler materials by having a similarchemical composition. For example, crushed alumina bodies made by themethod disclosed in U.S. Pat. No. 4,713,360, entitled "Novel CeramicMaterials and Methods of Making Same", which issued on Dec. 15, 1987, inthe names of Marc S. Newkirk et al., exhibit desirable infiltrationproperties relative to commercially available alumina products.Moreover, crushed alumina bodies made by the method disclosed inCommonly Owned U.S. Pat. No. 4,851,375, entitled "Methods of MakingComposite Ceramic Articles Having Embedded Filler", which issued on Jul.25, 1989, in the names of Marc S. Newkirk et al., also exhibit desirableinfiltration properties relative to commercially available aluminaproducts. The subject matter of each of the issued Patents is hereinexpressly incorporated by reference. Thus, it has been discovered thatcomplete infiltration of a permeable mass of ceramic material can occurat lower infiltration temperatures and/or lower infiltration times byutilizing a crushed or comminuted body produced by the method of theaforementioned U.S. Patents.

The size and shape of the filler material can be any that may berequired to achieve the properties desired in the composite. Thus, thematerial may be in the form of particles, whiskers, platelets or fiberssince infiltration is not restricted by the shape of the fillermaterial. Other shapes such as spheres, tubules, pellets, refractoryfiber cloth, and the like may be employed. In addition, the size of thematerial does not limit infiltration, although a higher temperature orlonger time period may be needed for complete infiltration of a mass ofsmaller particles than for larger particles. Further, the mass of fillermaterial (shaped into a preform) to be infiltrated should be permeable(i.e., permeable to molten matrix metal and to the infiltratingatmosphere).

The method of forming metal matrix composites according to the presentinvention, not being dependent on the use of pressure to force orsqueeze molten matrix metal into a preform or a mass of filler material,permits the production of substantially uniform metal matrix compositeshaving a high volume fraction of filler material and low porosity.Higher volume fractions of filler material may be achieved by using alower porosity initial mass of filler material. Higher volume fractionsalso may be achieved if the mass of filler is compacted or otherwisedensified provided that the mass is not converted into either a compactwith close cell porosity or into a fully dense structure that wouldprevent infiltration by the molten alloy. Through the dispersion of asecond matrix metal in accordance with the present invention, low volumefractions or particle loadings are also obtainable. Accordingly, a widerange of particle loadings can be achieved while still obtaining theprocessing advantages and properties associated with spontaneousinfiltration.

It has been observed that for aluminum infiltration and matrix formationaround a ceramic filler, wetting of the ceramic filler by the aluminummatrix metal may be an important part of the infiltration mechanism.Moreover, at low processing temperatures, a negligible or minimal amountof metal nitriding occurs resulting in a minimal discontinuous phase ofaluminum nitride dispersed in the metal matrix. However, as the upperend of the temperature range is approached, nitridation of the metal ismore likely to occur. Thus, the amount of the nitride phase in the metalmatrix can be controlled by varying the processing temperature at whichinfiltration occurs. The specific process temperature at which nitrideformation becomes more pronounced also varies with such factors as thematrix aluminum alloy used and its quantity relative to the volume offiller material, the filler material to be infiltrated, and the nitrogenconcentration of the infiltrating atmosphere. For example, the extent ofaluminum nitride formation at a given process temperature is believed toincrease as the ability of the alloy to wet the filler decreases and asthe nitrogen concentration of the atmosphere increases.

It is therefore possible to tailor the constituency of the metal matrixduring formation of the composite to impart certain characteristics tothe resulting product. For a given system, the process conditions can beselected to control the nitride formation. A composite productcontaining an aluminum nitride phase will exhibit certain propertieswhich can be favorable to, or improve the performance of, the product.Further, the temperature range for spontaneous infiltration with analuminum alloy may vary with the ceramic material used. In the case ofalumina as the filler material, the temperature for infiltration shouldpreferably not exceed about 1000° C. if it is desired that the ductilityof the matrix not be reduced by the significant formation of nitride.However, temperatures exceeding 1000° C. may be employed if it isdesired to produce a composite with a less ductile and stiffer matrix.To infiltrate silicon carbide, higher temperatures of about 1200° C. maybe employed since the aluminum alloy nitrides to a lesser extent,relative to the use of alumina as filler, when silicon carbide isemployed as a filler material.

Moreover, it is possible to use a reservoir of matrix metal to assurecomplete infiltration of the filler material and/or to supply a secondmetal which has a different composition from the first source of matrixmetal. Specifically, in some cases it may be desirable to utilize amatrix metal in the reservoir which differs in composition from thefirst source of matrix metal. For example, if an aluminum alloy is usedas the first source of matrix metal, then virtually any other metal ormetal alloy which was molten at the processing temperature could be usedas the reservoir metal. Molten metals frequently are very miscible witheach other which would result in the reservoir metal mixing with thefirst source of matrix metal so long as an adequate amount of time isgiven for the mixing to occur. Thus, by using a reservoir metal which isdifferent in composition than the first source of matrix metal, it ispossible to tailor the properties of the metal matrix to meet variousoperating requirements and thus tailor the properties of the metalmatrix composite.

A barrier means may also be utilized in combination with the presentinvention. Suitable barrier means may be required in the container 2 inwhich initial infiltration occurs, as well as in any mold into which thedispersed suspension is to be poured. Specifically, the barrier meansfor use with this invention may be any suitable means which interferes,inhibits, prevents or terminates the migration, movement, or the like,of molten matrix alloy (e.g., an aluminum alloy) beyond the definedsurface boundary of the filler material. Suitable barrier means may beany material, compound, element, composition, or the like, which, underthe process conditions of this invention, maintains some integrity, isnot volatile and preferably is permeable to the gas used with theprocess as well as being capable of locally inhibiting, stopping,interfering with, preventing, or the like, continued infiltration or anyother kind of movement beyond the defined surface boundary of the fillermaterial.

Suitable barrier means includes materials which are substantiallynon-wettable by the migrating molten matrix alloy under the processconditions employed. A barrier of this type appears to exhibit little orno affinity for the molten matrix alloy, and movement beyond the definedsurface boundary of the filler material or preform is prevented orinhibited by the barrier means. The barrier reduces any final machiningor grinding that may be required of the metal matrix composite product.As stated above, the barrier preferably should be permeable or porous,or rendered permeable by puncturing, to permit the gas to contact themolten matrix alloy.

Suitable barriers particularly useful for aluminum matrix alloys arethose containing carbon, especially the crystalline allotropic form ofcarbon known as graphite. Graphite is essentially non-wettable by themolten aluminum alloy under the described process conditions. Aparticularly preferred graphite is a graphite tape product that is soldunder the trademark Grafoil®, registered to Union Carbide. This graphitetape exhibits sealing characteristics that prevent the migration ofmolten aluminum alloy beyond the defined surface boundary of the fillermaterial. This graphite tape is also resistant to heat and is chemicallyinert. Grafoil® graphite material is flexible, compatible, conformableand resilient. It can be made into a variety of shapes to fit anybarrier application. However, graphite barrier means may be employed asa slurry or paste or even as a paint film around and on the boundary ofthe filler material or preform. Grafoil® is particularly preferredbecause it is in the form of a flexible graphite sheet. In use, thispaper-like graphite is simply formed around the filler material orpreform.

Other preferred barrier(s) for aluminum metal matrix alloys in nitrogenare the transition metal borides (e.g., titanium diboride (TiB₂)) whichare generally non-wettable by the molten aluminum metal alloy undercertain of the process conditions employed using this material. With abarrier of this type, the process temperature should not exceed about875° C., for otherwise the barrier material becomes less efficaciousand, in fact, with increased temperature infiltration into the barrierwill occur. The transition metal borides are typically in a particulateform (1-30 microns). The barrier materials may be applied as a slurry orpaste to the boundaries of the permeable mass of ceramic filler materialwhich preferably is preshaped as a preform.

Other useful barriers for aluminum metal matrix alloys in nitrogeninclude low-volatile organic compounds applied as a film or layer ontothe external surface of the filler material or preform. Upon firing innitrogen, especially at the process conditions of this invention, theorganic compound decomposes leaving a carbon soot film. The organiccompound may be applied by conventional means such as painting,spraying, dipping, etc.

Moreover, finely ground particulate materials can function as a barrierso long as infiltration of the particulate material would occur at arate which is slower than the rate of infiltration of the fillermaterial.

Thus, the barrier means may be applied by any suitable means, such as bycovering the defined surface boundary with a layer of the barrier means.Such a layer of barrier means may be applied by painting, dipping, silkscreening, evaporating, or otherwise applying the barrier means inliquid, slurry, or paste form, or by sputtering a vaporizable barriermeans, or by simply depositing a layer of a solid particulate barriermeans, or by applying a solid thin sheet or film of barrier means ontothe defined surface boundary. With the barrier means in place,spontaneous infiltration substantially terminates when the infiltratingmatrix metal reaches the defined surface boundary and contacts thebarrier means.

Various demonstrations of the present invention are included in theExamples immediately following. However, these Examples should beconsidered as being illustrative and should not be construed as limitingthe scope of the invention as defined in the appended claims.

EXAMPLE 1-4

The following examples illustrate the spontaneous infiltration of afiller material with a matrix metal, and the subsequent dispersion ofadditional matrix metal, to obtain a fully dispersed, homogeneous,pourable suspension, having a substantially lower particle loading thanthe undispersed spontaneously infiltrated composite.

FIG. 2 illustrates schematically the lay-up for Examples 1-2. Theexperiments for Examples 1 and 2 were performed simultaneously andside-by-side. For both of Examples 1 and 2, a 316 stainless steel can(101), 6 inches in diameter and 4.5 inches in height, was lined withPermafoil®, which functioned as a non-reactive container for spontaneousinfiltration.

In Example 1, about 300 g of filler (102) comprised of a mixture ofsilicon carbide (1000 grit 39 Crystolon from Norton Company) and about 2percent magnesium (325 mesh) was provided. An ingot (103) of about 600 gof an aluminum alloy containing about 12 weight percent silicon about 5weight percent zinc and about 6 weight percent magnesium (Al-12Si-5Zn-6Mg) was placed on top of the filler (102). A layer of 50 mesh magnesiumpowder was placed at the interface between the filler (102) and theingot (103).

In Example 2, about 300 g of filler (104) comprised of a mixture ofalumina (1000 grit E67 Alundum from Norton Company) and 5 percentmagnesium (325 mesh) was provided. An ingot (105) of about 600 g of astandard aluminum 520 alloy (containing 10 percent magnesium) was placedon top of the filler (104). Again, a layer of 50 mesh magnesium wasplaced at the interface between the filler (104) and the ingot (105).

Both stainless steel containers (101) were placed in a 14 inch long by 8inch wide by 7 inch high 316 stainless steel can (106), which wascovered with copper foil (108). A layer of Fiberfrax® (107) (McNeilRefractories, Inc.) was placed at the bottom of can (106) to insulatethe smaller cans (101) from the bottom of the furnace floor. A titaniumsponge (109) was placed along the bottom of the larger can to absorb anyoxygen which might enter the system.

A 2:1 ratio (by weight) of matrix metal to filler was utilized inExamples 1 and 2 to ensure that there was an excess of matrix metal, andthat a reserve of additional matrix metal would remain after spontaneousinfiltration was complete. The 2:1 ratio, after dispersion, was selectedto create a 33 percent (by weight) loading of particles to matrix metal.

The lay-up (100) was then placed in a furnace, purged with nitrogen viainlet (110), and heated from room temperature to about 800° C. over aperiod of about 2 hours under a flow of nitrogen gas at a flow rate ofabout 2.5 l/min. for approximately 2 hours until spontaneousinfiltration was substantially complete.

Cans (101) containing the spontaneously infiltrated composites werethereafter removed from the furnace at 800° C. and stirred immediatelyby hand in air for 2-3 minutes with an alumina stirring rod, which alsohad been heated to the furnace temperature.

Both composites mixed well. The resulting suspensions were then pouredinto a stainless 5 inch by 5 inch steel square frame mold set on acopper chill plate having water circulating therethrough at roomtemperature (22° C.).

The suspension produced according to Example 1 (the silicon carbidefiller) poured well and filled the shape of the mold. The suspensionproduced according to Example 2 (the alumina filler) poured as a lump,but showed moldable and extrudable characteristics. Both examplesdemonstrated the applicability of the dispersion method of the presentinvention to convert unmoldable and unpourable metal matrix compositeshaving particle loadings of the order of 50 percent to a pourablecomposite having a particle loading of the order of 30 percent.

The experiments of Examples 1 and 2 were identically repeated asExamples 3 and 4, respectively, except that a set-point furnacetemperature of 850° C. was used in an attempt to render the composites(e.g., the suspensions) more pourable.

The suspension of Example 3 was harder to stir and pour than thensuspension of Example 1. This diminished stirrability and pourability,however, may have been the result of more complete spontaneousinfiltration before mixing in Example 3 than in Example 1 resulting inbetter particle wetting. Example 4 did not show any change inpourability as compared to the alumina filler and matrix suspension ofExample 2.

EXAMPLE 5

This Example demonstrates the fabrication of a cast metal matrixcomposite material in a "two step" process. In a first step, a highlyloaded metal matrix composite is prepared by spontaneously infiltratinga matrix metal into a permeable mass of filler material and thereaftersolidifying the matrix metal. In the second step, the formed highlyloaded metal matrix composite is reheated and dispersed into the melt ofa second matrix metal. The assemblies used to carry out these two stepsare depicted schematically in FIGS. 3 and 4a-4d, respectively.

Specifically, in reference to FIG. 3, a filler material mixture 24comprising about 702 grams of 39 CRYSTOLON® silicon carbide (1000 grit,Norton Co., Worcester, Mass.), having an average particle size of about6 microns, and about 23 grams of magnesium powder (-325 mesh, ReadeAdvanced Materials, Rumson, N.J.) was ball milled for about an hour inan approximately 4 liter porcelain ball mill jar containing about 1450grams of about 1 inch (25 mm) diameter alumina stones.

A Grade ATJ graphite mold 20 (Union Carbide Corporation, Carbon ProductsDivision, Cleveland, Ohio) measuring about 5 inches (127 mm) long byabout 5 inches (127 mm) wide by about 21/2 inches (64 mm) high wascoated on the interior surfaces with a coloidal graphite aerosol spray22 (DAG 154, Acheson Colloid Co., Port Huron, Mich.). A total of fourcoatings of the graphite spray were applied. The coated graphite mold 20was then placed into an air atmosphere furnace and heated to about 380°C. at a rate of about 400° C. per hour. After holding at about 380° C.for about 2 hours to set the graphite coating, the furnace was allowedto cool naturally. Once the furnace temperature had dropped below 100°C., the coated graphite mold 20 was retrieved from the furnace.

The filler material mixture 24 was poured into the coated graphite mold20, levelled, and tamped repeatedly to pack the particles more closelytogether. A GRAFOIL® foil 26 (Union Carbide Corporation, Carbon ProductsDivision, Cleveland, Ohio) measuring about 5.0 inches (127 mm) long byabout 5.0 inches (127 mm) wide by about 0.010 inch (0.25 mm) thick andcontaining a hole 29 measuring about 1.5 inches (38 mm) in diameter wasplaced on top of the packed filler material mixture 24. Magnesium powder28 (-50 mesh, Reade Advanced Materials) was sprinkled evenly over thetop of the graphite foil 26 and the exposed filler material mixture 24to a concentration of about 40 milligrams per square inch (6.2 mg/cm²).Several ingots of a first matrix metal 30 comprising by weight about 12percent silicon and the balance aluminum and collectively weighing about1550 grams, were placed on top of the graphite foil, and morespecifically, around but not on top of the hole in the graphite foil 26,so that when the ingot melted, only fresh metal would come in contactwith the filler material mixture 24. The top of the coated graphite mold20 was covered with a second graphite foil 32, on top of which wassprinkled additional magnesium powder 34 (-50 mesh, Reade AdvancedMaterials).

The coated graphite mold 20 and its contents were then placed into astainless steel boat 36 measuring about 11 inches (279 mm) wide by about12 inches (305 mm) long by about 14 inches (356 mm) high. Magnesiumturnings 38 and titanium sponge 40 were also placed on the floor of thestainless steel boat around the outside of the coated graphite mold 20.A copper sheet 42 measuring about 15 inches (38 mm) wide by about 16inches (406 mm) long by about 15 mils (0.38 mm) thick was placed overthe top opening of the boat 36 and folded over the sides of the boat 36to form an isolated chamber. A purge tube 44 for supplying nitrogen gasto the isolated chamber was provided through the side of the stainlesssteel boat 36.

The stainless steel boat 36 and its contents were placed into aresistance heated air atmosphere furnace. The furnace door was closed,and a nitrogen flow rate of about 12 liters per minute was establishedwithin the stainless steel boat 36 through the purge tube at ambientpressure. The furnace was heated to a temperature of about 535° C. at arate of about 400° C. per hour, held at 535° C. for about 1 hour, thenheated to about 790° C. at about 400° C. per hour, and held at about790° C. for about 2 hours. During this time, the matrix metal alloyspontaneously infiltrated the filler material mixture to produce ahighly loaded metal matrix composite.

The stainless steel boat and its contents were retrieved from thefurnace at a temperature of about 790° C. and placed on a refractoryplate under a fume hood. The copper foil 42 and top graphite foil 32were removed and the still-molten carcass of matrix metal 30 was coveredwith an exothermic hot-topping particulate mixture (Feedol-9, Foseco,Inc., Cleveland, Ohio) to establish a temperature gradient duringcooling to directionally solidify the formed highly loaded metal matrixcomposite. Once a majority of the hot-topping mixture had reacted, thegraphite boat and its contents were transferred to a water cooled copperquench plate to maintain the temperature gradient. After cooling tosubstantially room temperature, the formed metal matrix composite andthe carcass of matrix metal were removed from the graphite boat, and thecomposite was separated from the carcass.

A second matrix metal ingot 202 weighing about 1419 grams and comprisingby weight about 12 percent silicon and the balance aluminum was placedinto a silicon carbide crucible 200 having an opening measuring about 6inches (152 mm) in diameter at the top, 3 inches (76 mm) in diameter atthe base, and about 8 inches (203 mm) high. The crucible 200 was thenplaced into the aforementioned isolated chamber which was fabricatedfrom a sheet of copper foil and a stainless steel can containing a gasinlet tube. The can and its contents were in turn placed into an airatmosphere furnace and heated to about 700° C. at a rate of about 400°C. per hour under flowing nitrogen gas. The furnace atmosphere waschanged to air once the furnace had reached about 700° C., as thefurnace was opened to remove the copper foil cover on the stainlesssteel can. Once the metal ingot 202 had melted, the surface dross wasscraped off from the metal ingot 202 and two irregularly shaped piecesof highly loaded metal matrix composite material 204, formed asdescribed above, were placed into the melt of the second matrix metal202. After the matrix metal in the highly loaded metal matrix compositesbecame molten, two more pieces of the highly loaded metal matrixcomposite material were added and their respective matrix metals melted,for a total addition of about 1556 grams of highly loaded metal matrixcomposite material. A preheated stainless steel rod 206 coated withcolloidal graphite (DAG 154 and measuring about 1/2 inch (13 mm) indiameter and about 24 inches (610 mm) long was then inserted into themelt and used to crush the highly loaded metal matrix compositematerial, all of which are shown in FIG. 4a. The coated stainless steelrod 206 was removed from the melt and, as shown in FIG. 4b, a fixture208 was then placed into the melt, said fixture 208 comprising a 11/2inch (38 mm) diameter stainless steel impeller coated with colloidalgraphite (DAG 154, Acheson Colloid Co.) and mounted to a 1/2 inch (13mm) diameter, 24 inch (610 mm) long shaft. The fixture 208 was rotatedat about 1550 rpm for about 2 minutes by using a lab stirrer (Lab MasterT51515 Mechanical Stirrer, Lightnin Mixer Co.) attached thereto (notshown in the figure) and located external to the furnace thereby forminga metal matrix composite suspension 210, shown in FIG. 4c, saidsuspension 210 comprising the former highly loaded metal matrixcomposite material now substantially uniformly diluted and fillermaterial therefrom being dispersed throughout the second matrix metal.The impeller fixture 208 was removed from the suspension 210 and thecoated stainless steel rod 206 was reinserted into the suspension toconfirm that the filler material agglomerates had been sufficientlycomminuted and dispersed. The coated stainless steel rod 206 was againremoved from the suspension and the metal matrix composite suspensionwas poured from the crucible 200, as shown in FIG. 4d, and cast into asteel mold (not shown in the figure) coated with colloidal graphite (DAG154 ) measuring about 6 inches (152 mm) square by about 2 inches (51 mm)high and which was situated on top of a copper plate. A sheet ofCARBORUNDUM® FIBERFRAX® ceramic cloth (The Carborundum Company, NiagaraFalls, N.Y.) measuring about 8 inches (203 mm) square was placed on topof the mold to insulate the top of the cast metal matrix composite in aneffort to achieve directional solidification. After cooling tosubstantially room temperature, the cast metal matrix composite wasremoved from the mold.

Subsequent optical microscopy on a polished cross section of the castmetal matrix composite revealed that the process of dispersing thehighly loaded metal matrix composite material into a second matrix metalreduced the volume fraction of silicon carbide particulate reinforcementfrom about 40 volume percent in the highly loaded metal matrix compositeingot to about 20 volume percent in the cast metal matrix composite.

EXAMPLE 6

This Example demonstrates that an aluminum metal matrix composite bodycomprising about 30 volume percent silicon carbide particulatereinforcement can be successfully cast by the methods of the presentinvention. Some representative physical properties of the cast body arealso included in Table I. The setup employed in fabricating the highlyloaded metal matrix composite is shown in FIG. 5. The setups employed indispersing the highly loaded metal matrix composite to form a suspensionof filler material and casting the formed suspension of filler materialto form a relatively lower loaded metal matrix composite material weresubstantially the same as those shown in FIGS. 4a through 4d.

In reference to FIG. 5, a filler material mixture 25 comprising about1500 grams of silicon carbide (500 grit, 39 Crystolon, Norton Co.,Worcester, Mass.), having an average particle size of about 17 microns,and about 45 grams of magnesium powder (-325 mesh), Hart Corporation,Tamaqua, Pa.) was ball milled for about an hour in an approximately 8liter porcelain ball mill jar containing about 3100 grams of about 1inch (25 mm) diameter alumina stones.

A GRADE ATJ graphite mold 20 (Union Carbide Corporation, Carbon ProductsDivision, Cleveland, Ohio) measuring about 6 inches (152 mm) long byabout 6 inches (512 mm) wide by about 21/2 inches (64 mm) high in itsinterior was coated on the interior surfaces with a colloidal graphiteaerosol spray 22 (DAG 154, Acheson Colloid Co., Port Huron, Mich.). Atotal of four coating of colloidal graphite 22 were applied. The coatedgraphite mold 20 was then placed into an air atmosphere resistanceheated furnace and heated to a temperature of about 380° C. at a rate ofabout 400° C. per hour. After holding at about 380° C. for about 2 hoursto set the colloidal graphite coating 22, the furnace was allowed tocool naturally. Once the furnace temperature had dropped below 100° C.,the coated graphite mold 20 was retrieved from the furnace.

The filler material mixture 25 was poured into the coated graphite mold20, levelled, and tamped repeatedly to pack the particles more closelytogether. A GRAFOIL® graphite foil 26 (Union Carbide Corporation, CarbonProducts Division, Cleveland, Ohio) measuring about 5.0 inches (127 mm)long by about 5.0 inches (127 mm) wide by about 0.10 inch (0.25 mm)thick and containing a hole 29 measuring about 1.5 inches (38 mm) indiameter was placed on top of the packed filler material mixture 25.Magnesium powder 27 (-50 mesh, ground, Hart Corporation, Tamaqua, Pa.)was sprinkled evenly over the exposed filler material mixture 25 to aconcentration of about 100 milligrams per square inch (16 mg/cm²).Several ingots of a first matrix metal 30 comprising by weight about 12percent silicon and the balance aluminum and collectively weighing about2290 grams were placed on top of the graphite foil 26, around but not ontop of the hole in the graphite foil 26, so that when the ingots melted,only fresh metal would come in contact with the filler material mixture25. The interior of the graphite mold 20 was sprinkled, as shown, withadditional magnesium powder 27 (-50 mesh, Hart Corporation) until aconcentration of about 16 mg/cm² was achieved.

The coated graphite mold and its contents were then placed into astainless steel tray 46 measuring about 10 inches (254 mm) wide by about12 inches (305 mm) long by about 3 inches (76 mm) high. Aluminum nitrideparticulate 37 (Advanced Refractory Technologies, Inc. Buffalo, N.Y.)and TI-LOY 97 titanium sponge 40 (Chemalloy Company, Bryn Mawr, Pa.)were placed on the floor of the stainless steel tray 46 around theoutside of the graphite mold to gather any residual impurity oxidizinggases which may be present in the retort furnace into which the mold andits contents were to be placed.

The stainless steel tray 46 and its contents were placed into a retortfurnace. The furnace door was closed, and a nitrogen flow rate of about15 liters per minute was established into the retort chamber. Thefurnace was heated to a temperature of about 225° C. at a rate of about400° C. per hour, held at about 225° C. for about 12 hours, then heatedto about 525° C. at a rate of about 400° C. per hour, held at 525° C.for about 1 hour, then heated to about 780° C. at about 400° C. perhour, and held at about 780° C. for about 3 hours. During this time, thefirst matrix metal alloy 30 spontaneously infiltrated the fillermaterial mixture 25 to form a highly loaded metal matrix composite.

The stainless steel tray 46 and its contents were removed from thefurnace at a temperature of about 780° C. and placed under a fume hood.The graphite boat 20 and its contents were transferred from thestainless steel tray 46 to a copper quench plate. The still-moltencarcass of matrix metal 30 was covered with an exothermic hot-toppingparticulate mixture (Feedol-9, Foseco, Inc., Cleveland, Ohio) designedto establish a temperature gradient during cooling to order todirectionally solidify the highly loaded metal matrix composite. Aftercooling to substantially room temperature, the highly loaded metalmatrix composite and the carcass of residual first matrix metal wereremoved from the graphite boat, and the composite was separated from thecarcass. Surface contaminants were removed by sandblasting.

A second matrix metal ingot 202 weighing about 1120 grams and comprisingby weight about 12 weight percent silicon and the balance aluminum wasplaced into a silicon carbide crucible 200 having an opening measuringabout 6 inches (152 mm) in diameter at the top, 3 inches (76 mm) indiameter at the base, and about 8 inches (203 mm) tall. The crucible wasplaced into a controlled atmosphere furnace and heated to about 750° C.at a rate of about 400° C. per hour under flowing argon gas. Once thesecond matrix metal ingot 202 had melted, the molten second matrix metal202 was degassed with argon for about one-half hour.

Several blocks of the highly loaded metal matrix composite eachmeasuring about 3 inches (76 mm) square by about 11/2 inches (38 mm)thick and weighing a total of about 1510 grams were placed into a secondsilicon carbide crucible in a separate furnace. The furnace was heatedto about 300° C. under an argon gas cover.

A Grade ATJ graphite mold (Union Carbide Corporation) measuring about 6inches (152 mm) square and about 2 inches (51 mm) deep was placed into athird furnace and heated to a temperature of about 450° C. under anargon gas cover.

When the highly loaded metal matrix composite blocks had reached atemperature of about 300° C., the furnace was opened and the highlyloaded metal matrix composite blocks were placed into the second matrixmetal melt 202 in the first furnace. After the contents of the cruciblehad remelted, a preheated stainless steel rod 206 measuring about 1/2inch (13 mm) in diameter and about 24 inches (610 mm) long was insertedinto the melt and used to crush the composite blocks, as shown in FIG.4a. The stainless steel rod 206 was removed from the melt and, as shownin FIG. 4b. A fixture 208 was then placed into the melt, said fixture208 comprising a 11/2 inch (38 mm) diameter stainless steel impellercoated with colloidal graphite (DAG 154, Acheson Colloid Co.) andmounted to a 1/2 inch (13 mm) diameter, 24 inch (610 mm) long shaft. Thefixture 208 was rotated at about 1500 rpm for about 3 minutes by using alab stirrer (Lab Master, Lightnin Mixer Co.) attached thereto (not shownin the figure), which was located external to the furnace. During thestirring operation, the argon gas cover was maintained, but thetemperature of the melt dipped to about 685° C. upon initial contactwith the fixture 208. The effect of the stirring action was to form ametal matrix composite suspension 210 comprising the former highlyloaded metal matrix composite material now substantially uniformlydiluted and filler material therefrom being dispersed throughout thesecond matrix metal. The impeller fixture 208 was removed and thefurnace temperature was increased to about 750° C.

The first silicon carbide crucible 200, containing the formedsuspension, was removed from the furnace and the metal matrix compositesuspension was then poured through a refractory screen (43 mesh,Pyrotek, Inc., Carlisle, Pa.) into a third silicon carbide cruciblepreheated to a temperature of about 450° C. This third crucible and itscontents were then placed back into the first furnace and reheated to atemperature of about 750° C. When a temperature of about 750° C. hadbeen reached, the graphite mold was removed from the approximately 450°C. third furnace and placed onto a copper quench plate. The metal matrixcomposite suspension in the third crucible was then immediately castthrough a refractory screen (55 mesh, Pyrotek, Inc., Carlisle, Pa.) intothe graphite mold and allowed to directionally solidify. A sheet ofGRAFOIL® graphite foil (Union Carbide Corporation) measuring about 8inches (203 mm) square was placed on top of the mold to insulate the topof the casting. After cooling to substantially room temperature, thecasting was removed from the mold.

Some physical properties, and the techniques used to measure thephysical properties, of the resulting cast metal matrix compositematerial are discussed in the following paragraphs.

QUANTITATIVE IMAGE ANALYSIS (QIA)

Volume fraction of filler, volume fraction of matrix metal and volumefraction of porosity, were determined by quantitative image analysis. Arepresentative sample of the cast metal matrix composite material wasmounted and polished. A polished sample was placed on the stage of aNikon Microphoto-FX optical microscope with a DAGE-MTI Series 68 videocamera manufactured in Michigan City, Ind., attached to the top port.The video camera signal was sent to a Model DV-4400 Scientific OpticalAnalysis System produced by Lamont Scientific of State College, Pa. Atan appropriate magnification, ten video images of the microstructurewere acquired through the optical microscope and stored in the LamontScientific Optical Analysis System. Video images acquired at 50× to100×, and in some cases at 200×, were digitally manipulated to even thelighting. Video images acquired at 200× to 1000× required no digitalmanipulation to even the lighting. Video images with even lighting,specific color and gray level intensity ranges were assigned to specificmicrostructural features, specific filler material, matrix metal, orporosity, etc.). To verify that the color and intensity assignments wereaccurate, a comparison was made between a video image with assignmentsand the originally acquired video image. If discrepancies were noted,corrections were made to the video image assignments with a hand helddigitizing pen and a digitizing board. Representative video images withassignments were analyzed automatically by the computer softwarecontained in the Lamont Scientific Optical Analysis System to give areapercent filler, area percent matrix metal and area percent porosity,which are substantially the same as volume percents.

MEASUREMENT OF FOUR POINT FLEXURAL STRENGTH

The four point flexural strength of the metal matrix composite materialwas determined using MIL-STD-1942A, Flexural Strength of HighPerformance Ceramics at Ambient Temperature. Rectangular flexuralstrength test specimens having dimensions of about 2 inches (51 mm) longby about 0.24 inch (6 mm) wide by about 0.12 inch (3 mm) thick wereused. Test Figure configuration B as outlined in Section 5.2 ofMIL-STD-1942A was employed. The four point test fixture was then placedonto the base of Model No. CITS 2000/6 universal testing machine (SystemIntegration Technology, Inc., Stoughton, Mass.) having a 500 lb (2225N)full scale deflection load cell. A computer data acquisition system wasconnected to the measuring unit and strain gauges in the load cellrecorded the test responses. The flexural strength test specimens weredeformed at a constant cross-head travel rate of about 0.51 millimetersper minute. The flexural strength and the maximum strain to failure werecalculated from the sample geometry and recorded responses with programsfrom within the computer.

MEASUREMENT OF ULTIMATE TENSILE STRENGTH (U.T.S.)

The tensile strength was determined using ASTM #B557-84 "StandardMethods of Tension Testing Wrought and Cast Aluminum and MagnesiumProducts". The geometry of the pin-loaded tensile bar is shown in FIG.6. The strain of the pin-loaded tension test specimen was measured withstrain gauges (350 ohm bridges) designated CEA-06-375UW-350 fromMicromeasurements of Raleigh, N.C. The tensile test bar was placed intothe gripping fixture on a Syntec 5000 pound (2269 kg) load cell(Universal Testing Machine, Model No. CITS 2000/6 manufactured by SystemIntegration Technology Inc., of Straton, Mass.). A computer dataacquisition system was connected to the measuring unit, and the straingauges recorded the test responses. The test specimen was deformed at aconstant rate of0.020 inches/minute (0.508 mm/minute) to failure. Themaximum stress, maximum strain and strain to failure were calculatedfrom the sample geometry and recorded responses with programs within thecomputer.

MEASUREMENT OF MODULUS BY THE RESONANCE METHOD

The elastic modulus of the metal matrix composite was determined by asonic resonance technique which is substantially the same as ASTM methodC848-88. Specifically, a composite sample measuring from about 1.8 to2.2 inches long, about 0.24 inches wide and about 1.9 inches thick(about 45 mm to about 55 mm long, about 6 mm wide and about 4.8 mmthick) was placed between two transducers isolated from room vibrationsby an air table supporting a granite stone. One of the transducers wasused to excite frequencies within the composite sample while the otherwas used to monitor the frequency response of the metal matrixcomposite. By scanning through frequencies, monitoring and recording theresponse levels for each frequency and noting the resonant frequency,the elastic modulus was determined.

MEASUREMENT OF THE FRACTURE TOUGHNESS FOR METAL MATRIX COMPOSITE USING ACHEVRON NOTCH SPECIMEN

The method of Munz, Shannon and Bubsey, was used to determine thefracture toughness of metal matrix composite. The fracture toughness wascalculated from the maximum load of Chevron notch specimen in four pointloading. Specifically, the geometry of the Chevron notch specimen wasabout 1.8 to 2.2 inches (45 to 55 mm) long, about 0.19 inches (4.8 mm)wide and about 0.24 inches (6 mm) high. A Chevron notch was cut with adiamond saw to propagate a crack through the sample. The Chevron notchedsamples, the apex of the Chevron pointing down, were placed into afixture within a Universal test machine. The notch of the Chevron notchsample, was placed between two pins 1.6 inches (40 mm) apart andapproximately 0.79 inch (20 mm) from each pin. The top side of theChevron notch sample was contacted by two pins 0.79 inch (20 mm) apartand approximately 0.39 inch (10 mm) from the notch. The maximum loadmeasurements were made with a Sintec Model CITS-2000/6 Universal TestingMachine manufactured by System Integration Technology Incorporated ofStraton, Mass. A cross-head speed of 0.02 inches/minute (0.58millimeters/minute) was used. The load cell of the Universal testingmachine was interfaced to a computer data acquisition system. Chevronnotch sample geometry and maximum load were used to calculate thefracture toughness of the material. Several samples were used todetermine an average fracture toughness for a given material.

MEASUREMENT OF INDENTATION HARDNESS

Hardness was measured on the Rockwell A scale of an indentation-typehardness tester (Leco Corp. St. Joseph, Mo.).

MEASUREMENT OF ABRASION RESISTANCE (LOSS OF MATERIAL)

Abrasion resistance was measured in accordance with the procedurespecified in ASTM Test G65-81 "Standard Practice for Conducting DrySand/Rubber Wheel Abrasion Tests". The specific test machine was auniversal Model 4850 abrasion tester (George Fischer Foundry Systems,Inc., Holly, Mich.).

MEASUREMENT OF APPARENT DENSITY

The apparent density of a sample is measured by first insuring that thesample is completely dry. The mass of the sample is determined asaccurately as possible. The sample is then placed into the samplechamber of an AccuPyc 1330 Autopycnometer (Micromeritics, Inc.,Norcross, Ga.). The autopycnometer automatically calculates the apparentvolume of the sample. Apparent density is found by simply dividing theapparent volume into the mass.

MEASUREMENT OF THERMAL EXPANSION COEFFICIENT

The thermal expansion coefficient was measured using an Adamel DI-24Lhomargy Dilatometer in accordance with the procedure set forth in ASTMTest E228-85 "Standard Test Method for Linear Thermal Expansion of SolidMaterials with a Silica Dilatometer." Specifically, a test specimenmeasuring no longer than 51 mm nor shorter than 9 mm in length, smallerthan 10 mm in cross section, and having parallel ends was placed on analumina support. An alumina rod which contacted both the linear voltagedifferential transducer (LVDT) and the sample similarly replaced thesilica rod described in the ASTM test procedure.

                  TABLE I                                                         ______________________________________                                        Room Temperature Properties of a Metal Matrix Composite                       Material Made in Accordance with Example 6                                    ______________________________________                                        Quantitative Image Analysis                                                                         28-32                                                   Volume Percent SiC                                                            Four Point Flexural Strength                                                                        286 ± 27 MPa                                         Ultimate Tensile Strength                                                                           173 ± 21 MPa                                         Fracture Toughness (K.sub.1c)                                                                       12.0 ± 0.5 MPa m.sup.1/2                             Elastic Modulus (resonance method)                                                                  120-124 GPa                                             Thermal Expansion Coefficient                                                                       14.0 ppm/K                                              Apparent Density      2.78 g/cm.sup.3                                         Indentation Hardness  38-40 R.sub.A                                           Abrasion Resistance (Loss of Material)                                                              0.29 cm.sup.3                                           (dry sand rubber wheel test)                                                  ______________________________________                                    

EXAMPLE 7

This Example demonstrates the fabrication of a shaped metal matrixcomposite material by casting a suspension of filler material and matrixmetal into an investment shell mold.

To fabricate the shaped metal matrix composite body, a highly loadedmetal matrix composite body was first fabricated. The setup employed inmaking the highly loaded metal matrix composite body is shown in FIG. 7.Specifically, about 1500 grams of a filler material mixture 25comprising by weight about 3.0% magnesium particulate (-325 mesh, HartCorporation, Tamaqua, Pa.) and the balance 500 grit 39 CRYSTOLON® greensilicon carbide particulate (Norton Company, Worcester, Mass.) having anaverage particle size of about 17 microns, was placed into a porcelainball mill having a capacity of about 8.3 liters (U.S. StonewareCorporation, Mahwah N.J.). About 4,000 grams of alumina based millingmedia, each having a diameter of about 1.0 inch (25 mm) was placed intothe ball mill. The filler material mixture was ball milled for about 2hours, and then poured into a graphite boat 20 having a wall thicknessof about 1/4 inch (6 mm) to 1/2 square by about 4.0 inches (102 mm)deep. The interior of the graphite boat had previously been aerosolspray-coated with about four (4) thin coats of AERODAG® colloidalgraphite 23 (Acheson Colloids Company, Port Huron, Mich.) and then hadbeen dried at a temperature of about 380° C. in air for about 2 hours.

The graphite boat 20 and its contents were then placed into a vacuumdrying oven and held at a temperature of about 225° C. for about 12hours to remove any residual moisture from the ball milled fillermaterial 25. The graphite boat 20 was then shaken to level the fillermaterial contained within and then tapped gently several times to packthe filler material particles more closely together. A layer ofmagnesium particulate 34 (-325 mesh, Hart Company, Tamaqua, Pa.) wasthen sprinkled substantially evenly over the top surface of the fillermaterial 25 until a concentration of about 0.1 gram per square inch (16mg/c²) had been achieved.

Several ingots of a matrix metal 30 comprising by weight about 12.0%silicon and the balance aluminum, and totaling about 2,286 grams inmass, were placed into a second graphite boat 21 whose interior measuredabout 61/2 inches (165 mm) square by about 4.0 inches (102 mm) deep andwhose wall thickness measured about 1/4 (6 mm) to 1/2 (13 mm) inchthick. This second graphite boat 21 also featured an approximately 2.0inch (51 mm) diameter hole in the base. The top opening of this secondgraphite boat 21 was covered loosely with a sheet of GRAFOIL® graphitefoil 32 (Union Carbide Company, Carbon Products Division, Cleveland,Ohio) whose edges were folded down over the sides of the second graphiteboat 21. The second graphite boat 21 and its contents were then placeddirectly atop the first graphite boat 20 and its contents and both wereplaced into a retort furnace. About 30 grams of aluminum nitrideparticulate 37 (Advanced Refractory Technologies, Inc., Buffalo, N.Y.)was placed into a refractory crucible 48 which in turn was placed intothe retort furnace adjacent to the stacked graphite boats 20, 21 to helpgather residual oxidizing gases from the retort atmosphere.

The retort was sealed and the retort atmosphere was then evacuated usinga mechanical roughing pump. The retort was then backfilled with nitrogengas to approximately atmospheric pressure. A nitrogen gas flow ratethrough the retort of about 15 liters per minute was established andmaintained. The furnace was then heated from about room temperature to atemperature of about 225° C. at a rate of about 400° C. per hour. Aftermaintaining a temperature of about 225° C., again at a rate of about400° C. per hour. After maintaining a temperature of about 525° C. forabout 1 hour, the temperature was then further increased to about 780°C. again at a rate of about 400° C. per hour. After maintaining atemperature of about 780° C. for about 3 hours, the retort chamber wasopened and the stacked graphite boats 20, 21 were removed to reveal thatthe matrix metal 30 had melted and spilled through the hole in the baseof the upper graphite boat 21 onto the filler material 35 in the bottomgraphite boat 20 and the matrix metal 30 had spontaneously infiltratedthe filler material 25 to form a highly loaded metal matrix composite.The second graphite boat 21 was removed from the first graphite boat 20and the first graphite boat 20 containing the formed highly loaded metalmatrix composite was placed onto a chill plate to effect directionalsolidification of the metal matrix composite body contained within. Theexposed surface of the metal matrix composite body was covered with asufficient amount of FEEDOL® No. 9 hot topping particulate mixture(Foseco, Inc., Cleveland, Ohio) to assist in maintaining the temperaturegradient during directional solidification. Upon cooling to about roomtemperature, the highly loaded metal matrix composite body was removedfrom the graphite boat 20 and was surface cleaned by grit blasting.

Next, the volume fraction of silicon carbide reinforcement in the highlyloaded metal matrix composite was reduced by forming a suspension offiller material in a matrix metal. Specifically, the suspension offiller material was formed by melting about 850 grams of a matrix metalcomprising by weight about 12.0% silicon, 0.03% strontium, and thebalance aluminum in a graphite crucible under an argon gas atmosphere ata temperature of about 750° C. The highly loaded metal matrix compositediscussed above was cut into several smaller blocks using a diamond saw.A number of blocks of the highly loaded metal matrix composite wereplaced into a second graphite crucible until a total of about 1,250grams of highly loaded metal matrix composite material were positionedin the crucible. This second graphite crucible and its contents was thenheated to a temperature of about 300° C. The preheated highly loadedmetal matrix composite blocks were then added one at a time to themolten matrix metal over a period of about 11/2 hours. The matrix metaland the added highly loaded metal matrix composite blocks were allowedto equilibriate at a temperature of about 750° C. for about 1/2 hourafter the last block had been added to the matrix metal. Theequilibriated mixture was then stirred for about 5 minutes at a speed ofabout 2000 rpm using a Lab Master T51515 Mechanical Stirrer which wasthe same as that discussed in Example 5. After this stirring operation,a suspension comprising molten metal matrix composite and a reducedvolume fraction of silicon carbide reinforcement was ready to be castinto preheated investment shells.

A refractory investment shell mold defining the shape for the cast metalmatrix composite was fabricated as described below.

In reference to FIG. 8a, a commercially available metal golf club head50 was first spray coated with Grade MS-122 fluorocarbon release agentdry lubricant 52 (Miller-Stevenson, Inc., Danbury, Conn.). GI-1000rubber molding compound 54 (Plastic Tooling Supply Company, Exton, Pa.)was then cast around the spray-coated golf club head 50 to form a firstrubber mold 54 inversely replicating the shape of the golf club head 50.After curing the rubber molding compound 54 in air for about 12 hours,the spray-coated golf club head 50 was separated from the first rubbermold 54. The insert portion of the metal golf club head 50 which was tobe replicated with a cast metal matrix composite material was thenelectro-discharge machined from the as-received metal golf club head toleave behind a cavity 60 in the golf club head 50 of the desired shape.The hollowed-out golf club head was then placed back into the firstrubber mold 54, as shown in FIG. 8b, and a polyurethane based polymermaterial (C-1511 isocyanate urethane casting resin, Smooth-On, inc.,Gillette, N.J.) comprising by weight and about 50 percent catalyst andthe balance resin was cast into the cavity 60 of the golf club head 50within the first rubber mold 54. After the cast polyurethane materialhad cured and hardened, the contents of the first rubber mold 54 weredisassembled to reveal that the cast polyurethane material had formed anaccurate positive model of the desired golf club head insert, asrepresented by the reference numeral 58 in FIG. 8c.

As shown in FIG. 8c, was spur 62 was attached to the polyurethane golfclub insert 58 and a second rubber mold 64 was cast around the insert 58and the wax spur 62 to form a negative image thereof. The role of thewax spur 62 was to form the gate or riser portion of the subsequentlyformed investment shell cavity. After curing the rubber molding compoundin substantially the same manner as was described above, thepolyurethane insert 58 and the wax spur 62 were removed from the secondrubber mold 64 and a positive image wax pattern of the golf club headinsert and its attached gate section was then made by casting Grade5550-K. GRN. FLK. molten was 66 (shown in FIG. 8d) (Yates ManufacturingCompany, Chicago, Ill.) at a temperature of about 110° C. into thecavity in the second rubber mold 64. As shown in FIG. 8e, after the wax66 had cooled to about room temperature the wax model 65 and itsattached gate section 67 were then removed from the rubber mold 64. Thewax model 65 of the golf club head insert and the attached gate section67 were then attached using molten wax to a styrofoam cup 68 which hadbeen coated with wax 70 to form an investment pattern. The shape of thestyrofoam cup 68 would later form the reservoir portion of theinvestment shell to be cast around the wax model 65, gate section 67,and cup 68.

An investment shell 72 was then built up on the surface of theinvestment pattern. Specifically, the investment pattern comprising thewax model 65, gate section 67 and styrofoam cup 68 were dipped into aslurry comprising about 29.6 % by weight NYACOL® 1430AT colloidal silica(Nyacol Products, Inc., an affiliate of PQ Corporation, Ashland, Mass.),about 65.1% HUBERCARB® Q 325 calcium carbonate (-325 mesh, JM HuberCorporation, Calcium Carbonate Division, Quincy, Ill.) havingsubstantially all particles smaller than about 45 microns in diameter,about 4.3 % 500 grit (average particle diameter of about 17 microns) 39CRYSTOLON® green silicon carbide particulate (Norton Company, Worcester,Mass.), about 0.6% VICTOWET® 12 wetting agent (Ransom and Randolph,Inc., Maumee, Ohio), and about 0.3% DCH ANTIFOAM® defoamer (Ransom andRandolph, Inc., Maumee, Ohio). The coated model was then dusted orstuccoed with dry 90 grit RANCO® SIL No. 4 silica sand (Ransom andRandolph, Inc., Maumee, Ohio). The wax model 65 and its developinginvestment shell 72 were then dried for about 1/2 hour at a temperatureof about 65° C. The dried investment shell was then dipped for about 2seconds into a bath of NYACOL® 1430AT colloidal silica (Nyacol Products,Inc., Ashland, Mass.). This dip-dust-dry-wet sequence was thenimmediately repeated two more times. Next, the coated investment patternwas immediately dipped into a secondary slurry comprising by weightabout 1 part REDIP® indicator (Ransom and Randolph, Inc., Maumee, Ohio),about 2 parts VICTOWET® 12 wetting agent (Ransom and Randolph, Inc.,Maumee, Ohio), about 56 parts distilled water, and about 274 partsNYACOL® 830 colloidal silica (Nyacol Products, Inc., Ashland, Mass.),and about 700 parts RANCO® SIL No. 2 silica powder (Ransom and Randolph,Inc., Maumee, Ohio) to yield a slurry viscosity corresponding to about15 seconds in a Zahn No. 4 cup. The slurry investment shell was thenstuccoed or dipped into a fluidized bed of approximately 30 grit RANCO®SIL B silica sand (Ransom and Randolph, Inc., Maumee, Ohio). Thestuccoed investment shell was again dried at a temperature of about 65°C. for about 1/2 hour or until the REDIP® indicator in the shell changedin color from yellow/green to deep orange. This second dip-stucco-drysequence was then repeated an additional five (5) times. No prewettingof the investment shell with colloidal silica between dippings in thesecondary investment shell slurry was required. After application of thethird secondary shell layer, the developing shell was wrapped with wirewhich served as a reinforcement. Several 1/16 inch (1.6 mm) diameterholes were drilled in the investment shell in strategic places to assistin venting gases from the wax during the subsequent autoclaving.

The coated investment pattern was then placed into a steam autoclave toremove the wax 66 from the surrounding investment shell. Afterautoclaving at a temperature of about 300° F. and a pressure of about100 psi (690 kPa) for about 4 minutes substantially all of the wax 66had been removed from the surrounding investment shell 72. Theinvestment shell 72 was then removed from the steam autoclave and placedinto a resistance heated air atmosphere furnace at a about roomtemperature. The furnace was then heated to a temperature of about 900°C. at a rate of about 800° C. per hour. After maintaining a temperatureof about 900° C. for about 1 hour, the styrofoam had combusted; anyresidual, impurity oxidizing vapors had outgassed and the investmentshell 72 had rigidized (e.g., some sintering occurred). Accordingly, thefurnace temperature was decreased to a temperature of about 750° C. Thefurnace was maintained at this temperature until the rigidizedinvestment shell 72 was ready to be used in the pressureless metalinfiltration process.

When the formed metal matrix composite suspension discussed above wasready to be cast into the investment shell 72, the investment shell 72was removed from the approximately 750° C. air atmosphere furnace, andwas placed onto a refractory plate. After the metal matrix compositesuspension had been adequately stirred, the mixture was immediatelypoured into the investment shell 72 up to approximately the levelindicated by the numeral 69 (shown in FIG. 8e) in the gate section ofthe investment shell. The gate section of the investment shellcorresponds to the region of the shell which at one point housed the waxspur portion of the investment pattern (i.e., the portion 67 shown inFIG. 8e). The remaining volume in the investment shell was filled ortopped off with pure molten matrix metal at a temperature of about 750°C. (i.e., matrix metal containing no ceramic reinforcement) comprisingby weight about 12.0% silicon, about 0.03% strontium, and the balancealuminum. The investment shell and its contents comprising the metalmatrix composite suspension was then placed back into the approximately750° C. nitrogen atmosphere furnace for about 15 minutes. The investmentshell and its contents were then removed from the furnace anddirectional solidification of the molten matrix metal contained withinwas commenced. Specifically, air was blown around the base of theinvestment shell while the exposed surface of the matrix metal wascovered with a hot topping particulate mixture (FEEDOL® No. 9, Foseco,Inc., Cleveland, Ohio) to help maintain the temperature gradient duringsolidification. After the investment shell and its contents had cooledto about room temperature, the investment shell was removed with lighthammer blows to recover the cast metal matrix composite golf club headinsert and its attached gate section and reservoir. The attached gatesection and reservoir were removed using a diamond saw and the outsideface of the cast metal matrix composite was lightly diamond machined torestore the corresponding detail of the metal original portion of thegolf club head.

FIG. 9 shows the finished cast metal matrix composite 90 as mounted inthe modified metal golf club head body 92.

EXAMPLE 8

This Example demonstrates utilizing the techniques of the presentinvention on a larger scale than the Examples previously described.Moreover, this Example also demonstrates the fabrication of a cast metalmatrix composite body by a metal matrix composite casting method whereinthe second matrix metal has a different chemical composition than thefirst matrix metal.

A highly loaded metal matrix composite was fabricated as follows and asshown in FIG. 10. A 500 grit silicon carbide particulate filler material(39 CRYSTOLON®, Norton Company, Worcester, Mass.) was admixed with amagnesium particulate (-325 mesh, Hart Corporation, Tamaqua, Pa.) insubstantially the same manner and in substantially the same proportionsas was described in Example 6. The admixed silicon carbide and magnesiumparticulates formed a mixture 25, which was vacuum dried for about 24hours at a temperature of about 225° C.

The vessel 80 used for housing the filler material 25 and matrix metalduring fabrication of the highly loaded metal matrix composite compriseda steel tube measuring about 51/2 inches (140 mm) in diameter by about 3feet (0.9 meters) long. The steel tube 80 was closed on one end and waslined on its interior surface with about 3 layers of GRAFOIL® graphitefoil 82 (Union Carbide Company, Carbon Products Division, Cleveland,Ohio). About 25 lbs. (11.35 kilograms) of the admixed filler material 25was poured into the lined steel vessel 80, levelled and settled.Additional magnesium particulate 34 (-325 mesh, Hart Corporation) wasthen sprinkled evenly over the top surface of the filler material 25until a concentration of about 16 milligrams per square centimeter ofmagnesium particulate was achieved.

The steel vessel 80 and its contents were then placed into a resistanceheated air atmosphere furnace at about room temperature. A nitrogen gaspurge tube 44 was supplied to the interior of the steel vessel 80terminating near the opening at the top of the vessel 80. Copper sheet42 was then placed around the purge tube 44 and the opening of the steelvessel 80 to effect a loose seal around the purge tube 44 and the upperportion of the steel vessel 80, but sufficient to maintain asubstantially nitrogenous atmosphere in the steel vessel 80 duringsubsequent heating. A nitrogen gas flow rate of about 30 liters perminute into the stainless steel vessel 80 was established. The gasexited at the loose fitting junctions where the copper sheet 42 met thepurge tube 44 and the lip of the stainless steel vessel 80.

The top cover of the furnace was replaced and the furnace was heated toa temperature of about 225° C. at a rate of about 400° C. per hour.After maintaining a temperature of about 225° C. for about 12 hours, thetemperature was then increased to about 750° C. at a rate of about 400°C. per hour. After maintaining a temperature of about 750° C. for about11/2 hours, the top cover of the furnace was removed, the copper sheetwas removed and about 22.7 kilograms of a molten matrix metal (not shownin the Figures) comprising about 12 weight percent silicon and thebalance aluminum was poured into the lined stainless steel vessel 80 ontop of the filler material admixture 25. The molten matrix metal beganto spontaneously infiltrate the filler material admixture 25 almostimmediately. The matrix metal was added in increments so as to be ableto gauge more easily the progress of spontaneous infiltration. After thelast increment of metal had been added, the stainless steel vessel 80and its contents were held in the furnace at temperature for about anadditional five minutes. Power to the furnace was then interrupted andthe furnace and its contents were allowed to cool back to roomtemperature.

The stainless steel vessel 80 was cut open and the layers of GRAFOIL®graphite foil 82 were removed to reveal that the matrix metal hadinfiltrated the filler material admixture 25 to form a highly loadedmetal matrix composite. The excess uninfiltrated matrix metal at the topof the casting was removed with a diamond saw and the exterior surfaceof the highly loaded metal matrix composite ingot was cleaned by gritblasting.

About 107 lbs. (49 kilograms) of a second matrix metal comprising byweight about 6.5-7.5 percent silicon, about 0.17 to 0.25 percentmagnesium, about 0.04 to 0.20 percent titanium<0.2 percent iron, <0.2percent copper,, <0.10 percent manganese and<about 0.10 percent zinc andthe balance aluminum (Aluminum Association Alloy No. 356) was placedinto a crucible measuring about three feet (0.9 meter) high and havingan opening measuring about two feet (0.6 meter) in diameter and heatedwith natural gas in air at atmospheric pressure. An investment shellmold having a shape and composition typical for the industry wasfabricated so as to cast a tensile bar shape. The investment shell moldwas heated to a temperature of about 593° C. in preparation for casting.When the approximately 107 lbs. (49 kilograms) of the second matrixmetal had melted, about 160 lbs. (73 kilograms) of highly loaded metalmatrix composite were added to the melt. When the contents of thecrucible had reached a temperature of about 650° C., a rod was insertedinto the melt to break down any remaining clumps of highly loaded metalmatrix composites. An air driven impeller which had been flame sprayedwith an alumina coating to minimize reaction with and contamination ofthe melt was inserted into the melt and the impeller was accelerated upto a rotation speed of about 200 rpm. After mixing the melt for about 10minutes at a speed of about 200 rpm with the air-driven impeller todisperse the silicon carbide filler material throughout the first andsecond matrix metals, the impeller was turned off and removed from themelt. After readjusting the melt temperature to about 650° C., a portionof the melt was immediately cast into the approximately 593° C.investment shell mold. The mold and its contents were allowed to cooland solidify naturally.

Once the investment shell mold and its contents had cooled to about roomtemperature, the investment shell was removed with light hammer blows toreveal the cast metal matrix composite body contained within. The metalmatrix composite body comprised the tensile bar pattern and its attachedgates and risers, as shown in FIG. 11.

A number of similar castings were made and the resulting cast metalmatrix composite bodies were sectioned using a diamond saw andcharacterized using the characterization techniques described in Example6.

The resonant frequency technique was used to determine the elasticproperties of the composite. The shear modulus ranged from about 45 toabout 48 GPa. The Yound's modulus ranged from about 120 to about 121GPa.

The bulk density was measured by dividing the mass of a rightrectangular specimen into its bulk volume as measured by its length,width and height. The bulk density ranged from about 2.74 to about 2.78grams per cubic centimeter.

Quantitative imagine analysis was used to determine the approximatevolume fractions of filler material porosity and matrix metal. Thevolume fraction of reinforcement filler ranged from about 26.6 to about37.7 volume percent. The volume fraction of porosity ranged from about0.9 to about 1.5 volume percent. The volume fraction of matrix metalranged from about 61.4 to about 72.4 volume percent.

The thermal expansion coefficient of a sample was measured in thetemperature interval between 50° and 150° C. at 50° C. intervals. Theaverage thermal expansion coefficient averaged over the three reportingpoints was about 14.2 ppm/K.

FIGS. 12a and 12b are optical photomicrographs taken at approximately50× and 400× magnifications, respectively, of a polished section of thecast metal matrix composite material.

EXAMPLE 9

This Example further demonstrates the formation and casting of a metalmatrix composite body having a reduced fraction of filler reinforcedmaterial on a scale larger than those described previously. Theprocedures employed in forming the suspension of filler material and thesubsequent casting thereof were substantially the same as thosedescribed in the previous Example except where noted below.

About 315 lbs. (143 kilograms) of the highly loaded metal matrixcomposite material, which was made by techniques substantially similarto those described in Example 8. was added to about 155 lbs. (70kilograms) of a melt whose composition was substantially the same asthat described in Example 8 (Aluminum Association Alloy No. 356). Theheat source comprised fuel oil. After remelting and dispersing thecontents of the crucible, additional Alloy 356 was added to the melt inan effort to reduce the melt viscosity and thus render the compositemore castable. After adding about 31 lbs. (14 kilograms) of additional356 Alloy and dispersing the addition into the melt, an additional 62lbs. (28 kilograms) of 356 Alloy was added to the melt and dispersed.Upon stabilization of the melt temperature at about 746° C., theviscosity was deemed to be acceptable and the metal matrix compositesuspension was cast into a number of standard green sand mold at atemperature of about 16° C. The interior of one of the molds defined afluidity spiral. Upon solidification of the cast metal matrix compositeand the further cooling of the composite to about room temperature, themetal matrix composite was removed from the mold and its surface wascleaned of debris by grit blasting the surface of the casting with steelslot.

FIG. 13 is a photograph of the formed fluidity spiral of the cast metalmatrix composite material.

A number of similar castings were made and the resulting cast metalmatrix composite bodies were sectioned using a diamond saw andcharacterized using the characterization techniques described in Example6.

The resonant frequency technique was used to determine the elasticproperties of the composite. The shear modulus ranged from about 41 toabout 51 GPa. The Yound's modulus ranged from about 120 to about 121GPa.

The bulk density was measured by dividing the mass of a rightrectangular specimen into its bulk volume as measured by its length,width and height. The bulk density ranged from about 2.67 to about 2.73grams per cubic centimeter.

Quantitative imagine analysis was used to determine the approximatevolume fractions of filler material porosity and matrix metal. Thevolume fraction of reinforcement filler ranged from about 26.6 to about37.7 volume percent. The volume fraction of porosity ranged from about0.8 to about 1.6 volume percent. The volume fraction of matrix metalranged from about 67.4 to about 73.7 volume percent.

The thermal expansion coefficient of two samples was measured in thetemperature interval between 50° and 150° C. at 50° C. intervals. Theaverage thermal expansion coefficient of the two specimens averaged overthe three reporting points was about 15.5 ppm/k.

FIGS. 14a and 14b are optical photomicrographs taken at approximately50× and 400× magnifications, respectively, of a polished cross sectionof another cast metal matrix composite which was cast and solidified inthis Example in a green sand mold having the same composition but ageometry different from that used in casting the fluidity spiral shape.

What is claimed is:
 1. A method for forming a shaped metal matrixcomposite body, comprising:forming a permeable mass with molten matrixmetal; spontaneously infiltrating at least a portion of the permeablemass to form a highly loaded metal matrix composite body; introducingsaid highly loaded metal matrix composite body and a second matrix metalinto a container having a cavity for retaining said highly loaded metalmatrix composite body and said second matrix metal; heating said highlyloaded metal matrix composite body and said second matrix metal to atleast the liquidus temperatures of the matrix metal of said highlyloaded metal matrix composite body and said second matrix metal to forma molten suspension; mixing said molten suspension to substantiallyuniformly disperse said filler material within said molten suspension;providing a mold having a shaped cavity therein; pouring said moltensuspension out of said cavity in said container and into said shapedcavity within said mold; and cooling said suspension to form a shapedmetal matrix composite body.
 2. The method of claim 1, wherein thecomposition of said second matrix metal is different from thecomposition of the matrix metal of said highly loaded metal matrixcomposite body.
 3. The method of claim 1, wherein the composition ofsaid second matrix metal is substantially the same as the composition ofthe matrix metal of said highly loaded metal matrix composite body. 4.The method of claim 1, wherein said mold having a shaped cavity thereincomprises an investment shell mold.
 5. The method of claim 1, whereinsaid highly loaded metal matrix composite body is introduced into saidcontainer at a temperature which is below the liquidus temperature ofsaid matrix metal of said highly loaded metal matrix composite body andsaid second metal is introduced into said container at a temperaturewhich is below the liquidus temperature of said second matrix metal. 6.The method of claim 1, wherein said highly loaded metal matrix compositebody is introduced into said container at a temperature which is abovethe liquidus temperature of the matrix metal of said highly loaded metalmatrix composite body and said second matrix metal is introduced intosaid container at a temperature which is below the liquidus temperatureof said second matrix metal.
 7. The method of claim 1, wherein saidhighly loaded metal matrix composite body is introduced into saidcontainer at a temperature which is below the liquidus temperature ofthe matrix metal of said highly loaded metal matrix composite body andsaid second matrix metal is introduced into said container at atemperature which is above the liquidus temperature of said secondmatrix metal.
 8. The method of claim 7, further comprising, after saidheating step, crushing or comminuting said highly loaded metal matrixcomposite body to assist in said mixing of said molten suspension.
 9. Amethod for forming a metal matrix composite body, comprising:forming apermeable mass of filler material; providing at least one of aninfiltration enhancer and an infiltration enhancer precursor to at leastone of a matrix metal and said permeable mass of filler material tocause spontaneous infiltration of the matrix metal, when made molten,into the permeable mass of filler material; infiltrating the permeablemass of filler material to a desired extent to form a highly loadedmetal matrix composite body; introducing said highly loaded metal matrixcomposite body and a second matrix metal into a container having acavity for retaining said highly loaded metal matrix composite body andsaid second matrix metal; heating said highly loaded metal matrixcomposite body and said second matrix metal to at least the liquidustemperatures of the matrix metal of said highly loaded metal matrixcomposite body and said second matrix metal to form a molten suspension;mixing said molten suspension to substantially uniformly disperse saidfiller material within said molten suspension; providing a mold having ashaped cavity therein; pouring said molten suspension out of said cavityin said container and into said shaped cavity within said mold; andcooling said suspension to form a shaped metal matrix composite body.10. The method of claim 9, wherein the composition of said second matrixmetal is different from the composition of the matrix metal of saidhighly loaded metal matrix composite body.
 11. The method of claim 9,wherein the composition of said second matrix metal is the same as thecomposition of the matrix metal of said highly loaded metal matrixcomposite body.
 12. The method of claim 9, wherein said mold having ashaped cavity therein comprises an investment shell mold.
 13. A methodfor making a shaped metal matrix composite, comprising:providing asubstantially non-reactive filler; spontaneously infiltrating at least aportion of the filler with molten matrix metal; supplying additionalmatrix metal to said spontaneously infiltrated filler to form asuspension, wherein said filler is substantially uniformly dispersed insaid matrix metal; providing a mold having a shaped cavity therein;pouring said suspension into said shaped cavity of said mold; andcooling said suspension to form a shaped metal matrix composite body.14. The method of claim 13, wherein an infiltrating atmospherecommunicates with at least one of the filler and the matrix metal for atleast a portion of the period of infiltration.
 15. The method of claim14, further comprising the step of supplying at least one of aninfiltration enhancer precursor and an infiltration enhancer to at leastone of the matrix metal, the filler and the infiltrating atmosphere. 16.The method of claim 15, wherein the infiltration enhancer is formed byreacting an infiltration enhancer precursor and at least one speciesselected from the group consisting of the infiltrating atmosphere, amaterial added to the filler and the matrix metal.
 17. The method ofclaim 16, wherein during infiltration, the infiltration enhancerprecursor volatilizes.
 18. The method of claim 17, wherein thevolatilized infiltration enhancer precursor reacts to form a reactionproduct in at least a portion of the filler.
 19. The method of claim 18,wherein the reaction product comprises a nitride of magnesium.
 20. Themethod of claim 14, wherein the matrix metal comprises aluminum, theinfiltration enhancer precursor comprises magnesium, and theinfiltrating atmosphere comprises nitrogen.
 21. The method of claim 13,wherein said matrix metal comprises aluminum and at least one alloyingelement selected from the group consisting of silicon, iron, copper,manganese, chromium, zinc, calcium, magnesium and strontium.
 22. Themethod of claim 13, wherein said shaped metal matrix composite has aparticle loading of about 5 through about 40 volume percent.